Well then it all depends on what you use it for.
If you want to really start duning Dumont then the KFX700 will be best. You can go fast OR slow on it and reach the tops of every hill there with no problem at all.
If you get a Rhino, you will be spending a bit more money and will be limited to where you go out there. You will have fun, but in time I think you'll get bored with it.
I'm not too familiar with your little buggy, but I'd say keep it for a secondary sand toy and definitley have it for the desert.