Honestly, I didn't like the Samsung Galaxy S2. The S3 is better, certainly award winning on specifications, but I'm not all that keen on the shape. It feels "plasticky". Maybe the S4 will be a step up.
I've long felt that GSM makes more sense than CDMA. I should be able to take out my SIM card and put it in any GSM capable phone to use it. But CDMA seems to have better performance results overall. If I was an international traveler, then I'd definitely go GSM. So far I've been happy with Sprint. The pricing is more advantageous than the competition overall, and I like how anytime minutes start at 7pm instead of 9pm (I make a lot of calls in the 6pm-9pm range). I picked up a free bonus 300 anytime min/mo added onto my plan earlier this year, too. Verizon, T-Mobile, and AT&T aren't very forthcoming with perks. These days Sprint seldom offers any, but something is better than nothing.