I need a cassette. I've got a 6700 group. A few months ago I got a set of nice wheels, and retired the ones that came with my bike, but I've decided I'd like to use them when I ride especially bad roads, and as spares. I don't want to move the cassette over every time I do this.
The 6700 cassette I've got is an 11-25, and a 10-speed. It has ~6,500 miles on it, and the wear checker tool makes it look like I can get a few thousand more. I used to get about 3K out of a cassette on my cross bike, so I'm happy with the longevity I'm getting now.
I know I can get rid of some weight (about 40 grams for DA, maybe 80 with Red) if I spend an extra $200. Which I'd like, but honestly wouldn't make much difference. I wound up really loving road riding, more than I expected, and if I did it all over again, I'd want to go with a lighter groupset than I have. Mostly just for vanity, though. Would a Red or DA cassette give me anything besides weight savings? Ultegra is already very good.
Also, I've been using an 11-25, and don't have any complaints about it, but I do a lot of climbing. If you were in my shoes, would you get the same gearing because it's worked fine, or would you get a few extra teeth just because?