Which tire / rim combo is better for auto-x?
 

225/50/15 on a 15x8

or

215/40/16 on 16x7.5

All other things equal, ignoring changes in gearing and tire compound, etc....

Which would have better stick and better turn-in characteristics?

All things being equal, you'd want to put as much rubber on the ground as possible. The exception being when the tire size starts to leave too little sidewall.

Street tires----16"

Race tires----15"

The stiffer sidewall of the race tires works better on a taller tire. Softer sidewall needs more rim imo.

I know Hoosier makes a 225/45/15 because I have a set.

I would also look for 13X8s if you can fit them.