I recently went through a purchase decision for a Grand L in this size range. I did go with the 3540 as comes in at 59.8" in width with R1 tires so I can use 60" implements and cover my tracks. Stepping up to the 3940, or even the 4240, was tempting for the extra HP and a 4 cylinder engine, but that frame comes in at 66.5" width with R1 tires which means getting 72" implements typically.
I didn't get a cab or hoe so I don't have the extra weight to carry around, i was picking R1 tires and I already had a number of 60" implements (tiller, discs, bush-hog). Each situation is different but I would definitely go for the higher HP for the frame size you pick - ie. the 3540 or the 4240.