4 casts VT
2.5 casts SWP
16 casts Flay
on average every minute
23 casts/minute at 15% proc rate is one Spellsurge proc every 20 seconds or so (internal cooldown 30 seconds) gives 200 mana every minute
Major Spellpower adds 1022 damage and 51 mana every minute
Soulfrost adds 1380 damage and 69 mana every minute
(these assume SF, Darkness, Weaving, CoS, Misery)
Really don't know what I messed up the first time, but there ya go. That's assuming perfect proc rate on Spellsurge (no overlaps, etc). Any Mind Blasts or SWD will probably favor Soulfrost, since the internal cooldown limits Spellsurge.
I'd definitely go with Soulfrost if you've got the mats.