I honestly don't get online often, and when I do, going onto a forum is even more rare. I have been a member on LVC for a long time and didn't even know this thread existed. I scroll down to Mark VIII, and that's where I stop. Anyway, I'm not sure how much you want to spend, but the cheaper 6X8 I had at one time were (Kicker DS68) which did sound pretty good. Yes, 6X8 and 5X7 work the same. I currently have (JL Audio Evolution C2 570X) Another thing you have to keep in mind is that some of us have aftermarket amps in our cars. That could be another reason people are looking at this, and then not replying to your question. If you have the factory JBL amp, which to me is weak, another speaker might sound better than the two I listed.
I would definitely change them out, because the stock speakers have seen their better days. There isn't much to them, except dry rot sitting on top of tiny magnets. Most of us that change our speakers out also have at least one 4-channel amp hooked up, and some, like me, have a sub amp and 4-channel amp. I have a 1200 watt for three 10" JL Audio subs and an 800 watt for the JL Audio interior speakers. All of my sound is coming a Sony XAV-AX5600
The speaker you choose to go with is going to be whatever you prefer. Look for good ratings, and research what people are saying about different 5X7 & 6X8 speakers. Good luck on whatever you decide.