Airvoice only does call forwarding for per minute plans.
As for where to port your number, it depends on what you're trying to do and where you spend the bulk of your time. In general, though, it will cost far less per minute to do simultaneous ring from a VOIPo account to your Airvoice phone.
As for the SMS question, it depends on where you port your number to. For the sake of argument, if you port your current phone number to VOIPo, your texting will be from the new Airvoice number for the most part. That said, VOIPo does have SMS support, contact a sales rep.
You need to make sure your LG Neon is carrier unlocked before you leave AT&T.