I would compare Stalker Complete to Killaps fallout mod, and O.L. to MIB88's megamod. Both good in their own respect, but where one focus on more purist things that "fit" into the game whereas the other adds content for the sake of fun. What you like most is a matter of preference, though i'd like to add that playing O.L. would make a wholly new experience whereas playing Complete would just fulfill and perfect the old feeling of the game. O.L. features blowouts though which is absolutetly fantastic as I recollect. The effect is MUCH better than CoP in fact.