My opinion only, ranked from top to bottom, of actual sets I have/had:
1-6500, nicely finished dual-pivot calipers with a smooth pull and even, consistent performance.
2-600 "tricolor" double pivots, about the same, maybe a little heavier.
3-105 single-pivot, polished. Toss on some decent Jagwire pads, and they're my best lightweights.
3a-DA 7400, with modern pads. Light like the 105's, well polished and reliable.
4-600 "tricolor" single pivot. A lot like the 105's, but dark anodized. Same deal, get better pads.
BIG GAP (I have new Chorus, new Veloce, DiaCompe BRS 200, BRS "Blaze," and Universal AER, all of which are better calipers, in my experience, than the 600EX. They're not ranked with the above because the thread is about those models)
15-600EX in polished or bronze. They look good, stop OK, but their QR's aren't that great and they don't work all that well with modern pads, which I use. The polished ones can look great with a chrome-accented bike, the bronze kind of need to be with the rest of a 600EX group.
My 2 cents worth, and I probably owe you change, at that...
Use both sides of the towel.
1985 Raleigh Racing USA - Competition (honoring cehowardGS)
1988 Centurion Dave Scott Ironman Master
1989 Centurion Dave Scott Ironman Expert
1989 Centurion Carbon R