So lots of threads about Columbus tubes on BF. To sum up: SL and SLX are same except SLX is a little heavier and stiffer because ends of tubes are reinforced with ribs. SP ans SPX are same except SPX is a little heavier and stiffer because ends of tubes are reinforced with ribs. SP and and SL are same except SP tubes are little thicker and stiffer. SLX and SPX are same except SPX tubes are a little thicker and stiffer. Frames with SLX are going to be a little stiffer and a little heavier (because of the ribs) than frames with just SL. Frames with a mix of SL and SP are going to be a little stiffer (because SP dt is thicker than SLX) and a tad heavier (mostly because they are bigger) than SLX. SPX frames will be a tad heavier and a tad stiffer, because of tube thickness, than the others. All are made of high quality steel (same alloy) and for a 240 LBS rider, SPX will make a very nice frame while SLX might be a bit flexy.

Edited to add: Frame quality is mostly about the builder, and I have no clue who among the three frame builders you mentioned had the best builders.