Your a young un and not too tall either!!!
I'd have a go at 20"!!
NS race bikes have longer TT's and better geometry so they are a lot more forgiving than any OS BMX.
Cruisers are easier and more suited to your MTB skills though.
Most tracks have practice days and they usually have loaners so that you can try then out!!
Pop along and have a go!!
And try 'em both out!!!