As a hunter I'm constantly trying to figure out which stats to add, especially the trade-offs between Agility, Attack Power and Critical Ratings. To facilitate that I've produced a comparison. I'm using the following assumptions:

AP / DPS: 14 for autoshot, 5 for Steady Shot (from WOW message boards)
Crit rating/%: 22.08
Agi / crit%: 40
% autoshots in regular rotation (steady shots for the balance): 70%
Crit dmg = 2x regular samage

To use this chart, figure out your normal base DPS (I have a DPS meter that tells me I tend to do about 400dps... since I crit 19.5% of the time that means about 2/3 of the DPS comes from regular hits). Thus I'm at about 280 base on this chart. For me, 1 crit point is worth about .75 agi and 1 agi is worth about 1.8 AP.

So what are important considerations? Agi v. AP is usually 1:2 for both gems and enchants (the green gems for a red slot gives you 12ap or 6agi). If your base DPS is above 354, use the Agi gems otherwise use AP. etc. etc.

The next question is which scope to put on my yet non-existent epic lvl 70 weapon. Well, it depends (once again) on your Base DPS but also on the attack speed. The former affects crit and the latter affects the +12 dmg (since it is per hit and the dps therefore is 12/speed). Finally the scope only helps auto (and multi) shots while the crit impacs steady and arcane shots. I'm again assuing that 70% of the shots are auto (and multi). That addition to the calculation means it is almost always worth it to get the crit.

The slower the weapon, the less benefit from the +12 dmg and the higher the DPS the higher the benefit from the crit.