For £300 you can get a very good bike second hand. You can't really have a bike that is too good. So just what ever takes your fancy, ask for pics and checking the condition can normally hint how hard its been ridden! 20'' is rode in all disciplines, street/comp/tgs, were as 24'' is biased more towards street riding more flowing, spiny, bmx'y lines. Best way to see what suits you is to get to a group ride and just have a go on other peoples bikes!