Makes sense to me. We are always recommending .22s to people so they can learn the fundamentals without all the heavy recoil. I don't see why the same principal wouldn't apply to shotgunning. With the 20-gauge you could get used to everything without developing a flinch because you're anticipating recoil.
Pretty big difference. Your .22 ammo is $17-$20 per 550 rounds. 20-gauge stuff is gonna cost about $7-$8 per 25 rounds for bird shot and $5-$8 per 5 rounds for buckshot.
Don't worry about the questions. If you don't know something, keep the questions coming. Usually I, like most people here, would recommend a .22 to learn on and get familiar with. However, if home defense is in the equation, I would go with the 20-gauge. It's mighty hard to beat a shotgun home defense.
Welcome aboard, and happy shooting.