In that case, I'd say you won't see a lot of difference between the S and P tbh. Go with the S to save cash and space alike.
If space is an issue and/or the extra money I'd agree... However, I'd argue if space isn't a concern and the extra money won't impact your build or savings otherwise (and if so there's cheaper rads out there than these that perform well) I would argue the P series seems to make the most sense actually. You get 3 extra ports, less flow restriction, more coolant capacity, slightly more thermal dissipation capacity (particularly if you can run push-pull also) and I'd think the slightly lower FPI (again, especially with push-pull) should also cut down slightly of fan noise over the rad... But that's just my thinking.
They seemed to really punch above their weight with that S class though, so getting pretty nitpicky with the P, but I feel like the P offers enough extra benefits with thermal capacity and ports, versus the X class, which seems to be much less of an improvement over the P series than the P series is over the S series and thus harder for me to justify unless you just really want extra rad in your build - in that case though I'd probably look at something like the SR2 for the low fan optimization, extra ports and super low flow restriction.
That's been my thoughts when looking at radiator options to fill out my v3000+ build and so I'm thinking SR2 480 at the bottom where space isn't an issue, a P480m at the front in push-pull with 28-30mm thick fans (won't work with the 58mm rad) or maybe a GTX 480 (hard to decide, but leaning P480m for the extra ports and lower flow restriction) and then either another 480m or X480m up top or maybe even another SR2.
So hard to decide, but that's been my thought process so far. With all that rad and tubing/fittings, flow restriction is important and thickness is nice where it's not an issue just to have it, but I feel the SR2 takes better advantage of it than the x360m.