I might as well go down in a blaze of glory on this one..
I chose the 4200rpm because I wanted more battery life. The 5400rpm will definitely yield more performance, but it also reduces battery life marginally. I'm mobile a lot so battery life is more important.
And, to be quite honest, I run Word, Excel, Netscape, iCal, Mail, Remote Desktop Connection, MSN Messenger, iTunes, and Terminal constantly, and I notice no performance issues.