You'll likely get this a lot but...
A good .22. It'll be easier to master good shooting with and inexpensive to shoot.
Between the two choices you've put in the title I'd say 9mm for the same two reasons. If you can manage the recoil better you're more likely to use better form, less likely to develop a flinch. Also, if you can find it, 9mm ammo is significantly less expensive.
Body type will be of less importance if you spend your up-front time learning good mechanics.