So you've decided to get a double barrel huh? To start with I'd say...put down the magazines, quit searching the net, stop fondling other peoples shotguns, the elusive grail of a double barrel is just not worth the anguish
...but if you still want a shotgun...
For $600 I'd look into a pump or an automatic. They are much less expensive and very reliable. A gas operated automatic has less recoil, while a pump is like very low maintenance and reliable. For $600 (or a little more) you can get a high quality used (and sometimes new-on sale/rebate etc) automatic, and certainly a new high quality pump. If this is your first shotgun ever, I'd stay away from the Benelli's or other recoil operated guns - they are good (I have one for my son), but they are expensive and kick like a pump, so if you can get a pump for hundreds cheaper, why not go with it...
Back to the doubles....I'm on my third...I've finally decided to just invest in a 'good' gun and see what the fuss is about. I forget the other inexpensive brands that I had in the past...I know one was a stoeger - gave it to my dad..I can't remember the other one. I shot both poorly...it could've been just me...I'm not a great shot, but I'm pretty darn good w/ my beretta.
My quest for a good double is not price...but the fit. If you hold the gun and close your eyes while swinging and then stop at a point and open your eyes to find yourself looking at the same thing the gun is pointing at then you win the poor man's fit contest. So I'd try that before you decided on a brand. I ended up with a browning o/u for about twice the amount you want to spend. They apparently have a good reputation in the clays community and it fits me good - so I bought it. I'll end up getting a sxs eventually too....maybe in a couple of years. Probably an Aya, beretta, or skb/weatherby - depending on fit.
As for your gauge, I'd choose the 12 unless you reload or want to hunt geese or something. 10 gauge shells are high (like 15.00 a box at walmart) and most stores seem to carry a large variety of 12 gauge shells. Cheaper shells mean more shooting, more shooting means more practice, more practice means being able to hit more things on the wing.
-Beetle