From what I have seen....the two clearest differences are 1) Carlos is probably a bit more versatile. He looks equally comfortable at right back, or right midfield. Pennant seems more like a pure winger. 2) Pennant is a better crosser of the ball, even though Carlos' crossing has improved quite a bit.
I would guess that Carlos is a better goalscorer, given his 9 league goals at Championship level vs. Pennant's 1 at Premiership level. Carlos seems to chip in with a few goals each season but he also gets more opportunities to play than Pennant...and of course, Carlos is scoring in the Championship not the EPL. Also, at 28, I think Carlos is closer to his peak than the 24 year old Pennant. In the end, Carlos is yet to prove himself at Premiership level, while Pennant has shown he can play with the big boys week in week out. But noone who has watched Carlos this season and at the WC last year can doubt he has the ability to make an impact at the highest club level..but talk is cheap...he still has to do it. Hopefully next season.