The difference will be lack of glance, sd. card slot, and shorter battery life. How much shorter is up for debate. Outside of that and the fact that the 1520 currently has cyan they are pretty much the same device.
Others opinions may differ from my own to include build materials used and experience with WiFi as well.
Nope, 930 rom cannot be flashed to a Lumia Icon. With denim, you get WP8.1 Update 1 features plus a faster Lumia camera with Denim
Someone said they put a T-Mobile SIM card in their icon, and said it updated their phone to denim. Does that make sense? Doesn't to me.