I'd say save up and get the 6GB model instead. But... that's me and my bias against the 3GB model. (It's more than a difference in RAM vs the 6GB model)
You're using this at work you say. Do they allow you to play games (in your free-time)? Do they allow you to modify their equipment, or is it your personal equipment? (Things to think of to stay out of trouble. You don't have to tell us the answer to these two - three questions.)
Either one will work fine. The Zotac might run a little warmer, maybe not. It might be a little louder, but it will work fine. The Gigabyte is more expensive and probably a little quieter. Both are good brands, so go ahead with the cheapest (the Zotac) if the noise is no concern.