I would choose the built-in Skype and true VoIP clients over Google Voice any day of the week.
As for LTE/4G you don't really need it unless you do lots HD video streaming
and if that is what you do on your phone most of the time than N900 is probably not your best option anyway.
Heck, I can't even get 3G on my phone with ATT, and still manage do most of my day to day tasks just fine.