Theory-Crafting

[capnpop]

Case 1: Shadow Mastery vs Shadow and Flame.

Shadow damage: if you use DoTs as a main source of damage Shadow Mastery wins hands down... S&F doesn't add anything to DoTs...for shadowbolt spam the two talents become roughly equal (assuming 5/5 in both of them) at around 500 +damage.  With the maximum rank of shadowbolt and +500 dmg SM will max out at 1136 (non-crit) and S&F will max out at 1133 (non-crit). Above that they remain relatively close (a test at +800 dmg showed S&F ahead by only 31 dmg at 1451) but I think the additions to DoT spells will eventually overtake that slight advantage(by a large margin i'm assuming as well).

Firelocks:  If you spam incinerate on anything that moves S&F is great.  First off: you almost NEED to take emberstorm (at least 4/5 for 8% more fire dmg) to get to S&F.  There are few good talents in that area that allows for the damage gains of emberstorm (nether protection i guess, but it's still only 3 points and rarely taken by many locks).  At 5/5 emberstorm and 5/5 S&F the +dmg coefficient is around 100% plus you get +10% to your base dmg of the spell AND +10% to the effect if immolate is on the target(meaning you get 100% return on your +dmg when casting incinerate)(2.5/3.5 = .7142 or 71.4% of + dmg  S&F = 71.4% + 20% = 91.4% * 1.1 = 100.57%). Total damage at max rank of incinerate with +500 dmg = 514 * 1.1 + 500 * 1.0057 = 1068.25 (add 111 * 1.1 if immolate is on the target).  At +800 dmg (and with immolate on the target) the maximum non-crit can be 1492.
If you don't have Immolate on the target it's only 1369.96. 

These numbers vary somewhat and they are all done with the max rank of the spells.  They are also assuming there is no other effect on the target that improves any kind of damage (Curse of shadow, curse of elements, shadow vuln, fire mage things, ect...) and it is also ignoring possible Imp Shadow bolt effects on the target.


Case 2: Incinerate vs Shadow Bolt

I'm an older warlock, when I started playing you were 30/0/21 or dead so my opinion is very biased toward shadow bolts.  Lately some warlocks have been going toward using incinerate as their primary form of damage in pvp, raids, everywhere.  In the above math it shows that when +shadow and +fire are equal, a target with immolate on it will take more damage from incinerate than from shadow bolt (and you are more likely to have a mage stacking the +fire stuff than a shadow priest stacking +shadow stuff...).  The unfortunate thing is that this is only true if there are 40+ points in the destruction tree.  If you skip out on 1 point in emberstorm or S&F the advantage disappears. 

Moral: if you don't go deep in destruction, stick with shadowbolts.  If you do go deep, it's still a tossup...