I hope I am following etiquette and that I am posting in the right group. What are the differences between these render farms and how can they benefit your project? Letís find out!

1. What is Rebus render farm?
RebusFarm is a German-based company that is part of the growing commercial rendering service for studios around the world. The company has a high capacity for data rendering with 3,000 XEON CPUs at their disposal. They offer a cloud rendering service that can handle almost every major 3D software out there. It is based on the model of SAAS (software -as-a-Service). You will submit your job directly from a plugin inside the 3D software. With the experience of more than 14 years in the field of 3D rendering, Rebus has been improving a lot to be one the the most trusted render cloud services. For CPU rendering, the render farm operates with AMDís Thread Ripper 3970X and for GPU rendering, they provide NVIDIAís Quadro RTX 6000 to render jobs on the cloud.
Below is a video of a typical work submission.

2. What is iRender farm?
iRender is a Vietnamese company operating internationally that is based on the model of IaaS (Infrastructure-as-a-Service), which rents the GPUs and CPUs on the cloud for 3D rendering. iRender was developed with the most powerful GPU and CPU infrastructure that can handle all the heavy-duty tasks like a breeze. The GPUs used are RTX 2080Ti/ RTX 3080/ RTX 3090 with heavy-duty processors (Dual Xeon E5-2670 & Intel Xeon Processor W-2245) having multiple cores to support the heaviest workload.
The simple to use rental service by iRender is the best, as it involves just a few steps to get started. First, register to the service, then create a new work environment or better known as a system image. This work environment will be saved and automatically loaded for the user as soon as logging into the account. After that, a remote desktop connection will be set up, and the user will be provided with a file with login credentials. The file transfer process is unique, it can transfer files even when the server is turned off, so the users can make changes any time. Furthermore, major cloud transferring platforms like Dropbox, Google Drive are also supported. Additionally, multiple servers can also be used, and the system image can automatically be generated, thus providing usefulness to the users.
iRender aims to facilitate everyone to access to the best and most powerful machines to turn their dreams into reality by providing their cloud render services for them.
You can even find the tutorial video here:

3. Render farm considerations
Render farms do have sets of parameters and limits, so it isnít always easy to compare companies. There are also important questions that you should have answered before hiring a service, such as:
What software & plugin does the render farm support? Is the render farm compatible with the software youíre using?
How much are you looking to spend?
Is the pipeline and workflow easy enough to be integrated and used?
What are the security, policy, and guidelines on the protection of privacy that they offer?
What are payment methods, claim and refund policies?
How about the customer support?
Some farms canít export in a file type compatible with a certain workflow. A render farmís highest priority is to save you time, so as with any processor-heavy project, be sure to do as much research as you can so that you can fully enjoy the benefits this type of service can offer.

I have actually used both render farms and I use 3dsmax, vray. I am an interior designer and have been using different render farms and I can tell you the differences and what will benefit you more.
When I used Rebusfarm it was nice, but I now know that it has some cons when using it. I sent a few renderings from my interior scenes and it was very expensive but since that was the time I started using render farms I was impressed with the speed and thought that the cost was normal. Then you have to have everything organized such as your textures, plugins, maps, and all your settings otherwise it wonít render. Then if there are problems with the rendering they will message you which is a good thing, but they wonít continue the render until you fix absolutely everything.
Now, donít get me wrong but Rebusfarm is good but I would say for smaller projects and if you donít have a tight deadline. As I have used them for large architectural projects and they were not close to the quote that was given with their calculator and during tight deadlines I have found that they arenít really good on response times with customer service even on their Germany working hours. So just be sure you know what you are getting into before sending your render to Rebusfarm.
Now iRender is a bit different from the average renderfarm that I find. Unlike Rebusfarm, it doesnít have a plug-in that you download and a client. iRender brings a cloud GPU rental service that lets the users make use of the powerful and robust features of the servers at iRender while using their own computers on their own desk. Users will access the servers through Remote Desktop Application, and then, take full control of the remote machines.
I can send multiple files for renderings at once and not have to worry about the rendering being stopped just because one thing is missing. Also, their customer service is much better and they get back to you almost right away. I have also found that when using iRender my costs are MUCH less than before and you can even see it for yourself. They are one of the lowest priced render farms and itís one of the reasons many people use them. I have also added one of their support teams Whatsapp numbers and whenever I need help right away she is able to reply me asap anytime and seems to be awake 24/7 and seems like she never sleeps but itís always the same guy. I also love the convenience where I can see the progress of the rendering at any time if I need to fix something and render it again.

These services are readily available to offer you the power and expedience you need to put your project, concept, or presentation together quickly and beautifully.
Honestly you can try both Rebusfarm and iRender farm to see what you most prefer, but my preference is iRender farm for a few main reasons. One of them being is the convenience of their client and the user interface is simple that anyone can use. Two, it is very cost effective compared to many other render farms and allows you to save a lot on rendering costs for future and many other renders. Three, I can see the process of my rendering to see if I need to resend the rendering. Four, you can contact customer support at any time to customize the hardware upto your needs. Five, you are able to run single card software like Lumion, Sketchup, Unreal Engine, so on, which canít be supported on other render farms. Six, the servers at iRender are highly available when you rent in days or week or month, users can access whenever they need without worrying about full servers or queueing in line like common cloud render farms. Next, as they donít charge you for the transfering files. Lastly, their customer service is top notch so if you have a deadline you can contact them and they will always be happy to help. So those are my reasons but I suggest trying both renderfarms and see what you find suits you best.