2.5 cas latency is faster. latency is a way of measuring how long it takes to respond. For instance, my girlfriend has a high latency. So, you want lower latency.
What is the computer used for? If it's high end and you're seeking performace, you should get the 2.5 latency ram. If you're just browsing and running light programs, ie, aim, minesweeper, then the 3.0 cas latency will do fine.
Hope I helped clear it up a little
Ryan