I shoot 60p...the only real difference is that you can go slower...once you pass a certain point the video will start to get choppy...unless of course you have a slowmo plugin like twixtor that guesses and creates intermediate frames for super slowmo
If your project is 25p I'm not really sure if 50p or 60p would be better. Maybe go with 50p since it's a multiple of 25 and as you say, it's smoother. In terms of shutter, I'd use the 180 degree rule and set it to double that of your frame rate. So 100 for 50p and 120 for 60p. If you're going to slow it down further in post, apply the same rule e.g. 100fps in post, set shutter to 200. I offer this advice having only read message boards on the matter and having no real experience. =)
Best bet is to do a few camera tests. Set your frame rate to 50p and try different shutter speeds starting at 100 and see what works for you. I plan to do this in the next week and will report back.