I just install my system and thinking using which speaker for my rearfill, i have a pair of apline 2way 6"5 speaker and a infinity 6x9. The infinity sound a lot better than the alpine but worry it will be too boomy, what do u guys think?

I tend to agree, 6x9's in an sq situation need to be faded down too low
i'd go the alpines, doesnt matter if they sound just great or if they sound amazing, cus they're just rear fill....
are u sure the 6x9s sound better than the alpine?? :/ *gods of alpine are condemming u atm*

Have you perhaps considered not adding any rearfill at all? This is what many people tend to do. Although personally i am a fan for a bit of rearfill. I've got some 4" Canton splits running of the deck. You can't really hear them at the front, but i think they add a bit more ambience to the overall sound.

Whether you choose the 6.5" or 6x9s, make sure you run them above 200hz or even higher. I would personally got he Alpines though.

I use rear speakers only when I have passengers in the rear! They're 5" Alpine splits mounted in the doors running off an amp, high passed at ~150Hz. Can't (really) hear them from the front, even though I run them slightly louder (gotta keep them balanced with the subwoofer sitting inches away from the rear seats:) Don't use them to add anything to the sound in the front; I just use them so that when I have people in the back, they don't leave the car thinking "jeez that sounded **** , all I could hear was bass" kind of thing.

i'd keep installation simple, and install whatever drops in easiest. there's not much point going to lots of trouble to fit some 6x9s when you won't run them to their full potential.

maybe the reason you prefer the sound of the infinity's is coz of the better bass. however, as mentioned, ideally they should be HP'd higher than the fronts, and 'faded' to your liking. so bass reproduction is not so relevant.

good luck

Id go with the Alpines if it were mine.