I understand where you're coming from, but you're asking us to use a crystal ball.
As far as updates are concerned, Samsung has said that the SGS will be updated to 2.2, that is the only info available.
GPS is a problem at the moment, but I for one hope that an answer will be found, either by Samsung or by the freelance developers.
My phone isn't laggy so I can't comment on that.
At the end of the day it's your money and therefore your decision, but all I can say is that I'm very happy with mine.