Infinity Kappa 6x8s and 6x9s or Boston Acoustics 6x8s and 6x9s?
the boston 6x9s have dual 1" tweeters and the 6x8s have a 3/4" tweeter.. The infinity kappa 6x9s have a midrange and 1" tweeter and the 6x8s have a 1" tweeter..
Suggestions please and if they have different sound which one is louder and which one is more sound quality...
Better is subjective. Both brands are going to offer a reliable product, the difference is going to be how they sound to you. If you can, you'll be best served to get out and listen to both sets. I'm personally a fan of BA over Infinity speakers, but that's just my opinion based on how I think they sound. There are plenty of people who swear by Infinity.
Loudness and SQ will depend largely on the rest of your setup and the quality of your install.
i would lean to BA only because my experience with inifinty has been they are way too bright. i have them in two cars and both i have problems with the highs being too piercing even with EQ toning down the highs. my experience with BA has been better, much more neutral sounding and better bass.
I would consider BA one of the top brands in that price range, and I'd say they're probably comparable to Diamond Audio speakers. I might rank MB Quart a little ahead of both, but then again you're going to pay a bit more for MB. It also depends on the specific series and speaker you're looking at. Value wise I'd still go with BA.