This completely depends on your current and desired stat distribution. Most people stick with Power crystals but there's no reason you can't use fewer Crit mods/enhancements with Crit crystals. This guide has some recommended stat levels (about 150 Crit, rest in Power). The current gear tiers (and most past ones) tend to give a ton of Crit so it often makes sense to take Power where you have the option.

Only hilts/barrels affect Weapon Damage so crystals, like other mods, purely add stats.

Since 4.0, the below has changed — you always want Crit instead of Power. See http://www.swtor.com/community/showthread.php?t=844210

This completely depends on your current and desired stat distribution. Most people stick with Power crystals but there's no reason you can't use fewer Crit mods/enhancements with Crit crystals. This guide has some recommended stat levels (about 150 Crit, rest in Power). The current gear tiers (and most past ones) tend to give a ton of Crit so it often makes sense to take Power where you have the option.

Only hilts/barrels affect Weapon Damage so crystals, like other mods, purely add stats.