I am guessing this is for a laptop.
This is really up to you and what you do. If you plan on doing large transfers i.e. stuff > 500mb a lot, then you should go for the 5400 RPM. Personally, i used to use a 4200RPM notebook HD and i couldn't stand it, so i upgraded it to the 5400RPM. Although it is a bit faster, it is still not comparable to my 7200RPM desktop. On my desktop, I could run virus scan, play counter-strike, run ventrillo, run aim, and other programs without lag. But on my laptop, everytime there is a virus scan, it does slow down my computer a bit.
I hope that helps.