I'd say w760i would be better, even though I like the looks of the w910i way more!

some things to keep in mind...

w760i
-GPS
-US 3G and 3.5g
-3.2 mp camera

w910i
-no gps
-no US 3G
-2 mp camera

both phones have the accelerometer and all the shake control stuff

here's a comparison on the UK site, the phones are the same, and I added my phone, the k850i, just for kicks. (love it by the way!)
http://www.sonyericsson.com/cws/products/mobilephones?cc=gb&lc=en#view=phones&cdl=1&s=0&detail=0&cmp=1&acc_cid=0&cid=0&acc_pid=0&pid=0&cmp_id0=262079&cmp_id2=255954&cmp_id1=339142