Okay, here goes.
As you said, these trucks are very diffrent. The X-Factor would be your best choice if you wanted some real life offroad action. It probably can crawl over any crevice or rock or things like that.
It is purely for slow motion offroading. It probably can reach high speeds because it has an immense engine. The 24.7 is a very big engine with awesome power and low end. The XF probably couldnt handle jumps and high speed bumps. Those things would probably blow right through the suspension.
As for the savage, that is in the same class as the Tmaxx. From what I've heard, not from experience, the Sav is a much better truck. Its plush suspension and its .21 capabilites are a big plus. As for size, it is only about 2 inches longer and alot taller, I think. But For racing, That truck is ideal. It is compatable with Tmaxx bodies wheels and rims, so that is a plus on your side. The savage also comes with HPI's .21 engine with (from what I've heard) isnt that bad. Its just not up to par with other .21's. The only downside to the savage is, its missing center protection. A useful upgrade would be a center skid plate.
As for my opinion, I would get the savage. But I dont own a MT and I dont know where to bash, so the savage would be a good racer if you ask me. The XF is probably the best realisitc MT out there. It is on the top of the rock crawling food chain. The XF has a serious ace up its sleeve, the R-Box. I dont know if youve heard of it, but its a electronic device where you can set a number of seconds, and after you hold the brakes for that long, it switches to reverse. Amazing! That saves you the hassle of getting that third channel radio and that little annoying button on the side.
As for your situation, If you are unhappy with your tmaxx, you could sell it and get the savage and upgrade it, or if your happy with tha maxxer, get the XF so you have two diffrent MT's.
Just my .02
[move]-Kellen[/move]