My boxed 6x9's are Kicker KS693. They are 3-ways and they put out really nice when wired to an amp. (Got em at 100watts RMS per channel, two channel amp)
Not sure how well they would sound running just off the stereo, being roughly 15-20 watts for a decent stereo. Titanium tweeters are nice and bright but for your application, the rear pillars, I think they'd be too harsh. I bought em cause they bear the elements better and metal tweeters are recommended for "out in the open" applications, such as a convertible.
No matter if you go with 4x6's or 6x9's, I'd recommend getting speakers with silk tweeters if you have a decent budget. Poly-anything-else tweeters would be second best. You probably don't want metal tweeters screaming in your ears at head-level and right behind you. Again, I'd go to Crutchfield.com and if you don't want to buy from them, at the very least, search and decide what you want based on their information. They have SO much more information than manufacturer and other retail sites have.