Hello,
Sound quality is the same between PGX and SLX. The difference between these 2 are the LCD display, # of simultaneous wireless devices, and antenna options. SLX/PGX is really not geared toward sound quality, but rather for basic speaking purpose. I would use them for announcement, but not for the pastor. The only good thing I can say about them is they have diversity RF, so they won't be interfered as easily if receivers are close to the mics. Put the receivers on the stage next to the stage snake, or hide them inside the pulpit.
ULX sounds better, but still not like a wired mic. I ended up pulling the trigger for a UHF-R system because the sound quality of that is close enough to a wired counterpart as far as my ears can tell.
For a little less money than ULX, Sennheiser EW-135/145/165 will give a better sound quality and rock solid reliability. Our church has a EW-165 for lead singer.
So you need 6 systems? If you are up to divide & conquer, get 2 high quality sounding systems, and 4 budget systems. For lead singer(s) get 2 ULX or 2 Sennheiser EW1xx, or the dual UR4D. For the rest, get 4 PGX's for back-up singers and announcements.