Billups - Iverson
Reason:Look what happen to the standing of both teams
DENVER more wins less loss
DETROIT more loss less win
Simple reason
Personally, I don't think that this is a "mismatch" per se, with due respect to AI, Iverson is a former MVP, averages 20+ points per game (in his whole 13 year career), he single- handedly brought the sixers to the finals a few years back. Billups is also good player, being a former finals MVP and champion. At the time of the trade, "In theory", it was good for both teams. The Pistons wanted to add additional fire power, and wanted to develop Stuckey (who the Pistons envision to be their future PG) and also cap space. They still have their veteran core of Wallace, Rip, and Taysaun. Denver on the other hand, wanted to have a veteran PG that would facilitate and lead the team. And Also they have their young core of Melo, Jr smith, Nene, and Kmart.
It just so happens that there were a lot of other reasons why they lost a lot this season (not JUST because of the trade), Detroit had personnel issues, aging veterans, injuries, and a new coach (who I think did a lousy job of controlling the team). And also, Rodney Stuckey didn't live up to their expectations (which I think was one of the main reasons why they lost so much).
On Denver's part, however, everything is working out great after the trade. But there are also other reasons. They are defending more, Nene is healthy and plays like a monster, JR Smith is shooting the lights out, even the Birdman is playing good ball. etc.
Lastly, Remember last year, Denver had a lot of injuries too and they still won 50 games (that's with Iverson on the lineup). Which means that Denver, at that time, was already good, It just so happens that all they need was just a veteran point guard. I read in an article in NBA.com, the writer pointed out that any elite veteran PG (Bibby, Kidd, etc) would still have the same effect on Denver if they were inserted into the line up, it just so happens that Billups was at the right place at the right time. I agree.
But hey, that's just my opinion.... peace!