High Yield, hands down. With good torps properly backed up by skills and consoles, often all you need to do is get the shields down: your torpedo barrage does the rest.
With Plasma torps, High Yield launches a ship-killing warhead that does absolutely stupid amounts of damage but its so dam slow and easy to shoot down. Otherwise it's far superior to Spread.
I don't even know if Spread is superior to High Yield in PvP - about all it's good for nailing cloaking Klings but if you had killed the target with High Yield in the first place, who needs Spread to finish them off =P
But as the poster above noted, it'll be looked at by the devs as a candidate for tweaking I think.
Torpedo Spread is actually useful in a few cases. I've noticed that for maneuverable enemies who rotate a weak shield away from you, the torpedo damage does damage to all facings, so it's useful for hitting that blind spot your normal torpedos cannot reach.
the spread needs some lovin - it needs to be faster or to be changed to some kind ot GT option that you are able to mark the place where it should datonate.
and on top its dmg should be equal to a shield less vessel compared to HY.