Frozen Orb- Mastery vs Synergy

[wsbchicago]

The topic is pretty self explanitory, I think, but let me ask anyway-

For a Sorc using Frozen Orb, which is better to invest points in: Ice Bolt for the Synergy, or Cold Mastery?

Assuming targets of varying levels of cold resistance, is the +%damage going to be more or less effective than -resists.  And where would the effectiveness of each skill overlap or exceed the other?

Thank you.

The only place where ice bolt would be better is when you are with a conviction pally and perhaps a necro with lower resist. If both of those spells are working, its possible that cold mastery would be redundant, thus useless. (It could have a good chance of being redundant in PVP, if you were on teams. On PVM, it would depend on monster resists.)