YES, SSM makes a huge difference. I've tried both the Sony 75-300mm (while waiting at Helsinki airport
) and the 70-200mm f/2.8 G SSM, and there's a gigantic gap between them. Not only in IQ of course, but the speed and accuracy of the autofocus is remarkable. It's just way, way faster than the body-driven AF. You really should go to a camera store and ask whether you can try an SSM/USM/HSM lens to see the difference. You'll like it, I'm sure.
How it works. Not 100% sure. Instead of relying on the AF motor build into the camera, there's a motor build into the lens, which gets the neccessary information from the camera and then focusses much faster and much more accurate.
It benefits all bodies, since the AF speed is no longer dependent on the body, only on the Super Sonicwave Motor of the lens.
Whether it's worth the money. If you buy the 70-300mm G SSM, it is, I guess. Combined with the higher quality of the glass. The 70-200 f/2.8 and the 300mm f/2.8 might not quite be worth the money for AF improvement only, the superb quality and constant high aperture are the biggest selling points for those. Same goes for the Zeiss 24-70mm f/2.8, also equiped with SSM. I'm interested to the price and performance of the 70-400mm G SSM coming up, that will be a very interesting lens as well.
Conclusion to this looooong post: if you have to money to buy SSM instead of non-SSM, get it. You'll enjoy it a lot, I'm sure.
I'm leaving for today! Good night y'all!
- Bjorn -