Their is a lot to be said for taking a recap of a particular fight or using /combatlog and a parser. Then sitting down with a calculator for 30 mins and doing the numbers based on the best real data you can establish for your gear and your damage.
spellstrike procs off any spell hit, which means it procs constantly when you're AEing so it's obviously better for that.
as for chain casting shadowbolt, with a 2.5 second cast time, lets add 500 ms lag, so 3 second cast time that's 20 spells per minute so 1 spellstrike proc per minute or an average of 15ish spell damage.
add to that a number of instant cast DoTs and your proc uptime improves a bit. for an affliction warlock I'd say there's no question that spellstrike is better if you're getting the 2pc bonus.
Indeed the Collar is (very) superior point by point. Having no red sockets and a +spell damage socket bonus is a bit annoying for me though. I would clearly shift from SS hood to this Collar if I got it - I'd also guess I might have found a legs slot improvement at this time, thus in summation upgrading well above the 2 piece set bonus too. Assuming that a cast rate of 3 spell hits in an average 10 second proc duration, SS set bonus proc boils down to 12 +dmg per spell hit (92*(3/23), 3 out of 23 hits, given that the 5% proc rate is correct). Scorch spam could ofc put you up to 6-7 spell hits in a 10sec period, but the lower +dmg coefficient would effectively bring it back down. Sorry if I'm being a bit fire mage specific here