I built the 1/72 Bradley that was reviewed and found it to be a fairly decent kit. The FSM review mirrored my own building experiences with it. There was some futzing around needed with a lot of the little pieces that comprised the turret, but all in all, it fit together well and was not especially difficult o build. Overall, it makes for a pretty good little Bradley. I an't speak about Dragon's 1/72 Challenger, but because the Bradley was a fun little diversion, I decided to pick up an Abrams to keep it company. I haven't built it yet, but out of the box, it looks to be a decent kit s well.
As far as the scale, take your pick. If you prefer 1/72 aircraft to 1/48, you might well enjoy doing 1/72 armor. Dragon's stuff in this scale is inexpensive, and fairly well detailed. On the other hand, if your preference for aircraft is 1/48, then 1/35 might be a better choice.