I have used both RT Motion and I currently own a full 3x Motor FIZ Chrosziel Magnum kit. By far the capabilities of the Magnum blows the RT Motion out of the water.
Their are other factors that do effect how you feel about the product and what your intended use is for it. If your clients/crew aren't wanting a Preston or Arri system then the only argument to make is functionality and features.
The nice thing about the magum is the motors are standard 7 pin lemo motors which means you can use just about any popular focus motor on the market from Heden (my personal favorite), Preston, Redrock, Bentz and Chrosziel even makes their own much which is fantastic.
RT Motion is a good working system that has never failed in use, but it is a closed system only RT Motion motors will work on RT Motions reciever.
They are both capable of 3 motors, but the Magnum hand unit has real hard stops built into it which depending on what you are doing can be an incredible tool. The knob also has the ability to adjust friction in the turn and the RT Motion 3.1 is locked at one levlel of fluidity which is a little more on the slow/harder to turn side.
The one feature that is hands down the best thing that no one else has is the spectrum analizer combined with the ability to do MHZ (with the attenna attached) and GHZ (via internal antenna. I was working in a courthouse doing B Roll with a steadicam operator for a documentary and the entire GHZ frequency had crazy interference from wifi and other wireless signals I'm guessing. I couldn't get farther than a few feet from the camera without the signal getting whacked out. Because of the MHZ function I was able to slap on the antennas and move on to get the shot. Aside from that if for some reason nothing is working wirelessly you can just connect a BNC/SDI cable from the reciever to the hand unit and it will work under any conditions.
I know RT Motion is popular, but I sat down at a rental house near me that has and compared the magnum with Preston HU3 & MDR3 and Arri WCU4 w/CForce Mini/Plus & CLM motors with MDR 1 to 1 with the most difficult to focus lens I could find (35mm T2.1 CP.2 Zeiss) and did a 1 to 1 test. I found that with Heden Motors the Magum system performed equally to all three systems. The only thing missing with the magnum is Lens Mapping (Which Chrosziel Says they are Working On) and exclusively with the WCU4 you can get wirless control of Arri Cameras especially with their Alexa Mini. The RT Motion like I said holds up well in performance it's just a closed system with service based in England (I could be wrong but last I heard service was in England.) Even though Chrosziel is a german company they use Radiant Images in LA to handle repairs so their is always someone in the US to help you if it goes down.
The final feature I love is the User Buttons you can program to do whatever you like and their is 4x I love saving time without digging through menus. Their are tons of other nice features, but these were highlights for me. Most of the times the first thing to wear out while in use is the focus motor. It's good to have backup and the fact that the Magnum takes any 7 Pin motor you will always be able to find something quickly to get you back up and running. The most important thing to remember is it's all about getting the shot. Whatever you choose make sure you can depend on it day in and day out.
If you get to know Timm Stemann you will always have someone you can call if you panic or need help. A generic 1-800 number the Redrock or any of the other guys can really suck.
Hope this helps! I have plenty of shots of the magnum in action on my instagram: @fortenox feel free to take a look.