I owned a .21 Maxx with an OS RG-X and a bunch of upgrades including CVD's all around and I now own a new 2.5 T maxx and I must say, on pavement or on light off roading, the 2.5 maxx feels like it may be faster or at least about the same plus the 2.5 can yank wheelies which make it more fun, my .21 Maxx of course had more torque and could pull harder in thick grass or heavy offroading, hmmm, for the $$ I'd definitely stick with the 2.5, you can get a new 2.5 and put a couple of upgrades and you'll be very happy, if you upgrade a Maxx to a .21, you will have to put in Cvd's and some other upgrades that make the truck heavier and weigh it down a bit plus it'll cost lots of $$, probably enough to buy two 2.5 Tmaxxes, so definitely go with the 2.5, it's very impressive and having owned a .21 Maxx, I think that's saying a lot, I am very happy with the 2.5 and do not intend to upgrade it to a .21, I just may have to go with that Sirio TX 18 a little later though...........