Now seriously… "If you're not very fit, do GT black and innerleithen red in one day". That's not advice, that's piss taking.
They're both ace… For some reason, it seems to be pretty popular on here to pretend that Glentress red is just a smooth path without anything to challenge anyone. Never understood that, when people say it's "smooth" I wonder if they've actually ridden there, there's parts of the red that look like they've been shelled. Innerleithen's surface is probably smoother in fact (less traffic) GT red's not tech, it's mostly built for speed and I think that's part of it, if you ride it slow there's not much challenge/peril.
For me, I think Innerleithen red is probably better, but GT red or black are both more fun. But it depends how you get your fun of course, we don't all want the same thing from a trail. I love all 3 mind. Doubt you'll regret whichever one you choose.