Hey! Glad to see another Sterling convert
Quality is pretty phenomenal across the board, so whether you pick up an older Sterling, or splurge on one fresh from the factory (contact Mike Taitt here on MCB for that, he's with Powerball, the current manufacturers/distributors of Sterling pumps, great to work with too!), performance is more or less a none-issue when it comes to narrowing down exactly what you want.
The only real difference is in the options:
Spyder threaded or Sterling threaded
Method of velocity adjustment (shims/adjustable bolt/adjustable hammer)
Right feed or center feed and/or stock feed
.45 frame or eurogrip or oldschool sterling frame
ball detent or no ball detent
backbottle or bottomlined
And I suppose old/new/newer pump handle style if you're feeling picky.
And for the most part, which you choose is up to preference. The only ones that I could really say make a big difference are the velocity adjustable hammer (makes things a lot easier at the chrony, and doesn't detract from effifciency like the adjustable bolt does) and the ball detent (because double feeds suck!).
Browse around the picture gallery and figure out what you'd prefer.