If it's the colours and RGB stuff you're after, definitely the 910. Otherwise, the two don't have much in common. The palm rest and the media buttons are just different. The rest depends on which buttons you choose.
If I didn't already have a 910, I'd go for the wired 815 or the wireless 915, but that's because I love the low keys, but I'm so happy with the 910 that it stays :)