It's going to be a 50/50 split; most people are going to go for the bottom end 27", but most people that you will speak to on the internet will go for the 27", with around just over half of those going for the higher end, non BTO model. This is just as a guess/what I've seen.
As for the 27" vs 21" debate, of course only you can make a judgement on whether you need it. However, you will definitely adjust to the 27" if you buy one. I found mine way too big at the start, but it really does seem normal now, the size isn't a problem at all. To be honest, it looks...medium size!