I'm probably a bigot here, but my opinion is, unless you want a really small, light shotgun, you should get the 12 ga.
You can buy (or make) 12 ga loads that have the same recoil as a 20. Plus you have the option of going to heavier loads if you ever decide to hunt waterfowl, etc.
A 20 ga is a good lightweight alternative to a 12 for smaller learners, but if you only have one, it should be a 12.