Which speakers will be better in front doors of my car, JBL GTO628 or GTO638 ?

The difference between a 2way coaxial driver and a 3way is the number of tweeters that are combined with the woofer. A 2 way speaker has one tweeter and one woofer, whereas a 3 way has two tweeters and one woofer. In a quality SQ system, the front speakers should dominate the soundstage, and with proper imaging the vocals should sound like they are in front of you. Rear speakers, again in quality sound systems, are generally used for rear fill, and some pros dont use them at all. I personally like components or a 3way coaxial in front, and a non dominant 2 way speaker, or none at all, in the rears. To answer your question, get the 3 way for the front, or if you can, a good set of component speakers. Comps will give you far better midrange and a better soundstage.