it depends what you're using the monitor for. A faster response time, will result in less ghosting (where you might see the image remain) while gaming and the like, while a 5ms ghosting will be more apparent, but it's not like drastic.
In my opinion unless you have a super eye for detail (or an imagination) you won't notice a difference.
I have 2 20.1" Benq monitors with a 8ms response and I don't notice any ghosting. Ghosting hasn't really been an issue with LCDs for the past few years, unless you buy a REALLY cheap terrible quality monitor.