Well, my vote is for SOG again. I have the Twitch I and it is fantastic. You can get the neatness of the spring assist with the reliability of a lock-back. Then their larger assisted knives, while not the nicest, can take an incredible beating.
Kershaw certainly has the widest line of assisted openers.
The action on the few Bucks I've handled were very nice, but I can't say I'm wild about the designs. A large, pointy safety just behind the flipper can be painful when actually using the knife.
I still love my SOG, though. Takes a licken, and keeps on cutting s~~~.
-KC