I've only had modafinil once (but it was amazing in the concentration-boosting department), but I have a lot of experience with caffeine, and the effects are primarily mood-affecting, for me. Large amounts of caffeine destroy concentration, offsetting any improvements, and, like other drugs, the effect grows weaker the longer you take it. On the plus side, caffeine is only weakly addicting, so you can just stop every now and then to reset things, which I do every few months.