Good point...dunno for sure, I have practiced double taps with both the 19 and 36 and personally don't see much difference in the two. Might be that I feel more comfortable with the grip and handling of 36? The 36 has a bit more recoil than the 19 of course but it's not nearly as snappy as my old G23 was.
Glock 19 for these reasons
1. Cheeper ammo (more practice)
2. Magazine capacity
3. 19's have a better track record of reliability
4. The ability to use different mags if you want to (G17&33 rounders)
5. If you have any interest in shooting sports IDPA, USPSA you'll be better off starting with a 19