In terms of passing, shooting, and court vision, Calderon
is the better PG. He can also surprise the defense and get to the rim for that little flip-in layup that he likes.
When it comes to rebounding, quickness, drawing contact, and defense, Lowry is better. Who is 'better' overall, though, really depends on how the rest of the team plays and what the match-ups are. I think right now, they're equally good at different things, so it's hard to make a definitive statement.
Personally, if Jose doesn't get traded by the beginning of the season, I think we finally have a pair of quality PGs who can compliment each other well. Lowry is for sure better than Jack, Ford, or Bayless, and a much better defender than any of them, which is good for the team. But I foresee yet another conflict emerging about minutes played, especially with how Lowry responded to Dragic.
Having said that, I really hope we can trade Jose by the deadline. By then, we and other teams in a similar position as us will know what moves they need to make, and whether making a playoff push makes sense. Besides, both Jose and Lowry are too good to be coming off the bench, so it only makes sense to use Jose and his expiring as trade bait to fill out the roster.