if you do street, with spins, manual, etc. i would go for high rise stem with short length, like a 25 degree, 90mm-100mm, if you do more trials, with less spins, more hops, hooks, etc. then go for a quit high rise (25 degrees) stem but just like 120-140, something like that i ride a 25 degree 70mm on a 24, i like to have it short and flicky to make spins easier (more bmxy) its all down to personal preference though hope this helps
chris