Infinity and Alpine both make decent entry level speakers but as is the case with any audio purchase, your best bet is to go out and listen to any speaker your thinking about. While Infinity and Alpine have similar build quality, they have VERY different sound characteristics. I personally prefer the sound Polk over Alpine and Alpine over Infinity, but that's just me. Ultimately, what sounds best to you is what you should go with, but any of those brands will be good in terms of quality of materials.
i think like ty i have had nothing but good luck with infinity reference series 5x7/6x8 i had a set in my truck then my t-bird and now my mustang and depending how they fit a set in my rabbit. but make sure they are covered my 3 1/2s surrounds got tore up by the sun really fast.