I'm the most picky comfort guy ever, and I can couldn't stand my DT770s for very long. They're comfortable pads, but lots of clamping force. You can stretch them out a bit though. Anyway, I've tried a700s before, and they're way more comfortable in every measure IMO. BUT, DT770s sound amazing.
(I recently got some Senn HD598 though, and they are really comfortable!)
Did you try the pro version? Because the premium version barely clamps once you stretch out the headband. They just sit snugly on my head, and are very comfortable for hours even though I wear plastic frame glasses. I've tried the HD600 in store before, and they are similiarly comfortable, except with an oval shape that might fit some ears better. So you won't go wrong with the premium beyers or with the HD600/650, just depends on your sound preference.
How would you describe the 880s sound? I'm assuming it's not exactly like the 770s, but I do love the sound and the comfort of my Beyers.. I was really only leaning towards Grado because even their cheaper options are well revered (60i/80i)
The dt880 sounds very similiar to the 770, but superior in almost every day. Since the dt880's are open, they will have a bigger soundstage and better seperation. They are also much more airy, with a very smooth treble with great extension. The dt770's treble is more peaky and is more prone to sibilance. The mids on the dt880's are also less recessed than on the dt770. As for the bass, the dt880 has less quantity, but more quality; it is very tight and clean, there is more texture to the bass, and you can hear the layering of it. The overall sound signature is very natural and linear, with a little bit of brightness. The brightness though, to me, is non-fatiguing and really brings out the details in my music. As such, the dt880 is great for pretty much every genre unless you listen to really bass-heavy music.
If you have a dedicated desktop amp, I highly recommend getting the 600ohm version. It isn't significantly different from the 250 and 32 ohm versions, but it has the smoothest treble and the best bass texturing as long as it is powered properly. What are you driving your headphones with?