To be honest, it doesn't matter too much mate. I believe the sampling rate just pertains to how many times it logs your depth for graphing purposes if you download your dives to a PC afterwards - there is no safety implication AFAIK.
The higher the sampling rate (ie 10sec as opposed to say 20sec), the fewer number of dives profile graphs you can download. I've had mine for a couple of years now and run it on 20sec. It'll quite happily store a week's worth of 2hr dives in this mode.
Basically what Rob Evans writes is correct. The sampling rate is the frequency of when the dive computer registers the info of the dive computer. The more often the sampling rate the more memory is used by the dive computer, therefore the fewer logs it will register. The advanatge of the higher sampling rate is that it gives a more precise graph.
WHat is important to note that the smapling rate does not affect the use of the computer in the water. The computer is working continuously, this is why there may be slight differences from the graph and what you noted in the water. For example if you have a sampling rate of 20 sec, and when these 20 sec you dropped down a further 5m and back up this would not be registered on the graph, but would show on the screen whilst doing so.
Personally I vary the sampling rate from 10 sec to 20 sec on my Cobra. If you are going to do many dives before downloading the info then increase the sampling rate, for example when you are going on holiday.