When to buy Sony.
If you're talking about Ring motor USM, than it is desirable. Sony is bringing that out now, and I have it on my 24-70, but it is more expensive, and, like Canon, it won't be on every lens. If you're talking about Canon's micro-USM, you can keep it. Screw drive is faster...sometimes it's the fastest of all. Micro-USM (aka pencil drive lens motor) is not good.