If I had the money I'd get the S6. That being, if I had the money that spending $500 seemed like nothing. Otherwise I personally would get the S5 and attempt to see if I could get a different pump kit on there.
Here's my opinion:
If you want an awesome pump marker...get the S5 Basic. You won't be disappointed with anything about it. CCM's Economy kit is head and shoulders above any other entry level pump kit.
If you want the absolute best marker out there, get the s6. That's what I shoot. That's what my teammates shoot, and I wouldn't have it any other way.
These two guns have the same lower tube internals, so they function the same way. The 6 just has more bells and whistles.
If you can find an SS-25, they're great guns and very versatile, but the valve system is a different style than the other two guns. This makes them a little different to work on.