I'm a fan of the swivel form factor. Battery life is good depending if it's new or not. I still use the current battery that I originally had. It's just that since it's been used for a while, it gets drained faster. I just got a new battery, and it's been good. The camera on the S710a is still one of the best, so that's a strong point for me.
You can't use Pro Duo, only the regular Memory Stick Duo. Some people told me that the nothing can be done for the phone to recognize Pro Duo's with hex editor because it's a hardware issue. But I'd rather find out for myself. So I got a 2nd S710a to work on.
It would be hard to compare the W810i and the S710a. Clearly, the size is very different. The UI on the W810 is more up to date. And of course, it does have a 2.0MP. There are alot more differences. just read up on the reviews and such on some sites. But if I had to pick one, I'd go with the W810i. If anyone else is wondering, yes I'd choose this even over the K790a (which I had used for about 2 months), just because it suits my needs more.
In any case, either choice you make is pretty well off.