My personal thoughts are, buy the size of bike which you want to end up on. Each size has its own unique style of riding and feel, not to say that you can't ride a 20" like a 24"!
I'm also 5"7 and started on a 26", rode that for about 3 years and got to a fairly decent standard then got a 20" and really vastly improved.
I have to say riding 20" for me personally is so much more enjoyable than 26" and I would recommend it to anyone. The manoeuvrability and flexibility of them is just too much of an advantage in my eyes.
I will say though I've never tried a 24" and wouldn't mind giving it a try, although I doubt I'd ever switch