The Volt II and OSO Sweet are pretty similar. Same materials, same size, and both assisted openers. The Volt II is a bit more stout (thicker liners) than the OSO Sweet, and the OSO sweet has a very slightly longer blade (maybe by 1/16th of an inch). Both are a bit larger than the Drifter.
I'd also toss the Cryo in there for consideration, if you want a stainless steel frame lock (or the G10 version, which has a G10 non-locking scale). The Cryo is closer in size to the Drifter. It too is a bit more stout (thicker handle slab and frame lock), and it offers more versatility in carrying and handling options. The Cryo has a quad-position deep carry clip and can be opened via flipper or thumb studs. It's also got a few nice extra touches as well, like the lockbar stabilizer, and a hollowed-out stand-off that doubles as a lanyard hole.
The Drifter is a nice, slim, and simple frame lock knife. It is right-side, tip-down only, so if that matters to you, it's worth keeping in mind. There is a G10 variation as well which has a G10 non-locking scale.
Kershaw's customer service and warranty are great. I've never dealt with CRKT's warranty service.
Here are a couple shots of those four...
And one showing the difference in the liners between the Volt 2 (left) and OSO Sweet (right):