Surround speakers: Better ones at rear or side?

Obviously, your front stage is the most important like limomangus said, but I'm assuming you're only asking about surrounds.

Side surrounds speakers should be better than rear surrounds.  Side surrounds still has some direct sounds generated (in movie soundtracks).  The rear surrounds are more for ambience and reflective sound.

Better speakers up front ,rears don't  have to be as good.The mains will be needed to be well the main.The rears more sound effects .

My suggestion is to place the better speakers closer to your ears (the side speakers), and the lesser-quality ones farther from your ears (rear wall). If you can aim the rear speakers to bounce off the ceiling, it will diffuse the sound more, and make the quality difference between the speakers less detectable.