Do you like a "bright" tweeter, or a "mellow" tweeter? All things considered nearly equal with the lower end; tweeters are what will fatigue you fater than any other speaker. If possible stay away from mylar tweeters the material isn't strong, and breaks up quickly. Metal dome tweeters are more revealing; but can be harsh to many ears. Silk dome tweeters are much more mellow than metal domes.
Look for speakers with high sensitivity. They will help you get the most out of the power offered by the radio you choose. If you find a nice set of 6x9's that would only afford you to buy that one set; than forget about the rear speakers and go for the nicer 6x9's. I promise you will not miss not having the rears.