I went from S0 SS to S2 SBSS.
Speed -- I don't really use apps but insofar as I do I've not noticed any speed improvements. Actually, now that I write that, I do remember having to wait sometimes with a spinning set of circles and feeling like an idiot with my arm in the air and eventually giving up with the S0. That's never happened with the S2. Maybe it's quicker and seamless now so that I'm already taking it for granted.
Tracking -- I love the tracking. I like seeing the map of where I've been.
Brightness -- again, I've not really noticed an improvement, but maybe it's subtle. Or maybe it hasn't been called into use. I live in the UK. We get sun about three days a year. The screen is beautiful and bright and clear.
Scratches -- After 18 months my S0 SS looked like the back of the old iPod: covered in dozens of micro scratches. After 1 month of the S2 SBSS, it's as perfect as the day I got it.
One factor you didn't mention:
Look -- I think the SBSS is so much better looking. It's beautiful and black and provides a perfect background for the beautiful bright screen. The SS gives the screen a silver border, which I personally don't like.
No regrets here, but it did cost me about £350 to make the change. The S0 didn't sell for as high a price as I'd hoped.
