In my prior vehicle, I had 5-1/4 Component Infinity Kappas. I've used Infinity Kappas in other installations and generally LOVE them. They're typically a little bright on the high-end, but tuning/crossover adj. can make them sound very nice.
However, I was very disappointed in their mid-range. Just too weak for my taste and the low-end was basically nonexistant (to be expected). I had a 10" Infinity Kappa Pro Sub w/480watts for the low end. The mids on the 5-1/4's couldn't play cleanly with any freq. below 150-180 Hz. There's a lot of important stuff if that freq. range. Likewise, the sub isn't intended to play that high a freq. without sacrificing it's performance.
I would definitely stick w/the 6.5's, if you can. But, this is just one opinion.
Good luck!