THere was an identical thread to this one only a week ago. CSA is newly manufactured with top quality fit and finish. Comes with only 2 mags but has a tabbed bolt carrier and can be had with either wood or plastic furniture and a pistol grip/M4 style sliding stock. The 858 is surplus, painted in an ugly color, not well finished on the outside. Bolt carrier is not tabbed so gremlim MAY exist (may not though). Comes with 4 mags and is only availiable with wood furniture in fixed or folding stock.
IMO, the 58 is worth the extra if you want a gun you can show off and be super proud of. If you just want a really fun gun to bash around and shoot off lots of surplus the 858 will do you just fine.