What do you want ?:

- A beginners heli ?: then the 30 or 50 are both good. The 50 will give you all you'll ever need.
- A heli you can upgrade ?: then you'll need or the 30 or the 60 (30 can be upgraded to 50, but not so good and 60 to 90, this one is extremly good.
- A good heli: then either the 30 or 50 will do (but again, the 50 will give you more power and you'll have everything you'll ever want in a heli. But offcourse later on you can still get a 60)

hope this helps.
if you got any more questions, just ask. (but know what you're starting with ... )