Yeah, haste is just better than crit for the reasons already covered.
Two sets (stam/vers and haste/vers) would be ideal. Personally, I haven't done so yet and have just tried to optimise my set around haste/vers and trying to maintain a good quantity of int.
In most situations (BG's etc) you're going to be wanting to get as much rot damage on the enemy team as quickly as possible. Haste is obviously going to help you do that more quickly and vers is simply more damage + survival.
A stam/vers set would be likely be more useful in duels/arenas, where additional survivability (however marginal) can make a yuge difference.
In BG's/Wargames, your positioning is ultimately much more important than tankiness. Since warlocks have zero real survival cooldowns, escape abilities, or oh shit distance creation (if you discount fear that's easily preventable and broken), you're pretty much dead if some DPS decide to switch to you in the middle of a fight, even with maximized stam/vers.