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Most posts I've read people are leaving the rears unplugged as they do not add to the sound stage. I am in the process of building up my sound system and I am on the fence on this subject as well. I know if you fade the volume all the way to the back in the stock system, you can barely hear those speakers.
What seems to make sense to me for rear fill you could use something cheaper (although something that can handle 120W RMS won't be that cheap) rather than installing a high priced speaker that doesn't add much at all to your overall sound.
I like rear fill. Gives me that 'wall of sound' feel. I also like to play it loud at times, the extra speakers allow that better than just fronts. I use JL XR-525 coax to compliment my JL C5-650 components up front.
Just because your amp can put out 120w doesnt mean you have to or should give them that much. Turning the gain down on the rears gives headroom and quality sound.
Matched components as in Vibe Space series in front/Vibe Space series in the back. Definitely coax for the rears. It would be a waste to put a nice set of components in the rear just for filler. You could go with their Slick series 5 1/4 coax but to me they don't seem to be on par with the Space series. There are quite a few higher quality coax out there. Out of your list I would go with a Polk MM521.