Few hours ago I got 640 for my dad. Before setting it up for dad I played with for a while and I find 730 camera way better than 640. However, I've tested it only indoors. Didn't want to go outside in 40 degrees.
Screen in 640 is not full clearblack like 730 and other higher range devices. It uses only one layer of polarizers.
Also, capturing pics is faster in 640 but focusing is much faster and accurate in 730.
Only thing 730 lacks is glance screen. So I'd go for 730.
This is 640 sample
This is from 730
