Being the proud (and VERY satisfied) owner of a G21, I would say: Either of them!
And I would like to refute the comment made earlier about Glock accuracy. A buddy of mine made a similar comment, then I took him to an outdoor range and popped 30 out of 30 on a stell silhouette at 50 yards. That was with my G21 AND my G19. I figure to never need to make a shot with a handgun at that range, but Glocks are more than "combat accurate" in my book.
Also, I think you'll find that you'll pay more than an additional $100-150 for the P220 over the G21. It'll be more like $200-250. But that shouldn't be enough to sway your decision, in my opinion.
As was said earlier: Try them both. One of them will "feel" better in your hands. Unless I miss my guess, any range worth going to will have both of them available for rental.