If you are unfamiliar with mining (and using GPU) I'd say GUIMiner since it's very easy to use. Don't expect to gain too much from mining without ASIC tho.
You want to tell me that GPU mining is not gonna make me much BTCs?
It might have given you lots of BTC for 2-3 years ago, but not anymore (especially not if you're on a nvidia card). With that said, I still suggest you try out mining to see exactly how much you are able to mine and see if it's worth the electricity.