Personally I would say 9mm, for precisely the reason you stated, price difference. 9mm is cheaper, cheaper ammo = more practice. Other than that, it really depends on your personal preference. For instance, are you going to be carrying or is it just for home defense? Depending on your build, weight, etc, if you are planning to carry you may be able to hide the G30 better than the G19...really depends on your size I suppose.
Other than that, I'd say either would be a good choice. I've shot both and like them both. I have to say, my friend's G30 is fun to shoot....that sucker puts out a pretty big fireball for such a little package.