I use both of those lines from time to time, but I mostly use the p-line for is a leader(I use braid exclusively in saltwater). I really love both lines. From my experience, which isn't a whole lot, as long as the water isn't clear enough you can see fish at 20', the line color doesn't matter so much. I don't think you can go wrong with either line. The p-line Floroclear has a little more stretch and doesn't sink as much, whereas the seaguar will sink and will stretch considerably less.
Will it be a main line?
What kind of baits are you planning to throw on it?
Proper Preparation Prevents [censored] Poor Preformance