Have owned both a 250t RS Four and (currently) a 350RX Four.
Go the 3.5. Interior (everything) is much nicer. Exterior is nicer. Transmission is nicer. AWD system is easier (no need to worry about external pumps). Engine is common although the 2.5t is much better for cheap performance.
3.5 is a better drive in factory spec. if you are modding, anything up to 6-7k will see significant performance increases with the 2.5t and okay increases with the 3.5.
If you are spending big for performance ($15k+), you will get more out the 3.5 but it will require stronger internals.
Neither have to be "boring" (brakes, suspension and bracing perk up the drive quite a bit) and both make an excellent kiddy wagon.