I skipped the 308 because I wanted a meter to use for non flash also. The 308 IIRC lacks aperture priority readings. The mode you pick an F/stop and the meter tells you the shutter speed. I think it also lacks a few other minor things but I may not be remembering correctly. The 358 wasn't much more at the time. Maybe $20 and did everything but spot. Today the 558 adds even spot.
The USB cable can be used for film curves too. I think it's the sort of thing some one with years of expierence does in their head. I plugged the cable in once or twice to see if it worked then decided it was too much effort to figure out at the time.