Agree that you should go with a new number. The reason that you don't get all the features when you use your existing number is because you are only forwarding calls to google voice when unanswered/unavailable. Google has announced their intentions of ultimately allowing you to fully 'port' your existing number over to their service (meaning that incoming calls will actually go straight to google voice, rather than through your home/cell carrier), which will ultimately allow all features, but that is not available yet.
As for the having to press 1 to answer or 2 to send to voicemail, I believe (though haven't checked) that this is configurable. I for one, could not live without the call screening feature however.