I've never tried the red ones, then again I've only just put SPD-SL's on my winter bike. I tried SL's a few years ago but didn't like them for longer rides, but I only have a short commute and they were cheap so I got them again
I'm used to Speedplay Zero's set with a bit of float, (the float is adjustable on Zero's), so the yellow SL's feel ok too me, but now I'm thinking I should have red ones, argh!