I'm a big p.s. fan as domestic travel goes, but since your alternative would be entirely international travel, it's probably not a huge difference . . . you can let United give you the p.s. sales pitch at http://www.unitedps.com . (You certainly do get some nice perks in p.s. first.)

As far as connecting at JFK . . . you are guaranteed not to have to walk far, since it's a small terminal and the United gates are close together, and it's generally comfortable (the United lounge there is pretty average; not great, not awful--although the chairs in the general boarding lounge are leagues nicer than in most airports). The biggest concern is possible delays, which are probably a given unless you're flying at a really off-peak time of day.

I'd say take the direct flight and save yourself the hassle and chance unless any of these things apply to you: you really want to try p.s.; you find some strange enjoyment in making otherwise unnecessary connections at big airports assuming all goes well; you have a strong preference for the 777 across the pond. Personally I feel the tug of all three