I just purchased the 7780 from a 7280 after hearing that the 7780 would have better RF. I haven't had the 7780 long enough to see if that is really true.
As stated previously, the backlight on the 7780 is a lot brighter and I like the black colour (although I don't like having the Rogers logo on the back of it).
I do like the form factor of the 7280 too, however I got a deal on the 7780 that I felt I couldn't refuse.
I'm selling a 7280 for anyone that is interested. Look in my signature below.