without knowing what else your running I dont know what else is on the 12v line but I would go for the 3.5 personally. Loads cheaper much faster (if that maters as its really the OS that needs the speed) and by the time you start adding multiple drives the extra power is negated. This assumes you have the 12v to spare. Infact if it was me (and I have) I would toss the 2.5 and get a 300 gig 3.5 then you will use less power. Just use ghost to copy your OS over and you wont have to re-build. If you dont have the power then go 2x 120 2.5's. The 2.5's are a bit more shock resistant but I have never had a problem with my 3.5 and its dead cheap to replace if it breaks anyway and way faster.