Well I'm a complete beginner and honestly I have no idea of wich one of the two will be better.
Wich one you think it is overall a better speaker and wich one it will pair better with my mains?
What is a back box?
I wish I could give you a straight answer. I've never heard either. Only in-walls I've ever heard have been bargain crap installed by people more concerned with architectural aesthetics than with accurate reproduction. But the Hsu bookshelf speakers on which the Hsu in-walls are based are very well-regarded for their price. And Atlantic Technology's stuff is likewise well-reputed.
Regarding the back boxes, they're simply boxes you build around the back of the speakers. You basically just add an upper and lower horizontal stud between the existing vertical studs, and put a little acoustic insulation (foam, fiberglass, whatever) behind the speaker. See
http://www.audioholics.com/diy-audio/in-wall-speakers/step-3-installing-the-speakers
Another option would be on-wall speakers. 2x
NHT Absolute Wall Speaker would be a worthwhile choice. It's simpler to install if you aren't comfortable with the in-wall back box thing while only sticking out about 4" from the wall.
Bipolar surrounds, like the
Fluance FLBPW for example, might also be worth considering. They are better suited for movies than for 5.1 music, but they are likewise designed to be mounted directly on the wall. If you're looking for more accuracy, the NHT's would probably be the better choice. I believe Definitive Technology and RBH Sound also make respectable on-walls, but I'm not sure whether they'd be available within your $300-ish target price for a pair.
Maybe one of these other guys will have other bright ideas?