Both the CZ and the Mossberg are Turkish guns. The quality will be about the same. I've heard both good and not so good about both but I would lean more towards the CZ just because I think they look a little nicer and they have a great warranty.
It kinda bothers me when people say their budget is limited. Okay, my budget is limited too, but it does not limit what I buy, I just save longer. If you really want a good OU, you would be better off waiting for a used Browning to pop up somewhere at a decent price.
Here's another thing (devil's advocate). You may do just fine (or well enough) with a Turkish made gun .A $2,000.00 Citori won't make you a better shooter, it will help good shooters hit more clays. The difference between a Browning and a Mossberg, for an average shooter, will be hitting 16 out of 25 instead of 15 out of 25.
Here's what's going to happen if you buy a Turkish gun. You will shoot some clays, you will have fun, and you will constantly be thinking "gee, I wish this was a Browning.". The difference in price between a new CZ and a used Browning will be anywhere from 300 to 500 depending on the grade of the Browning. Most folks that own Brownings take care of them, so it's not likely you will find a lemon.
As W2MC suggested, OU is not the end all be all of clay guns. Many trap shooters use semi-autos to great effect. One nice thing about a semi-auto is you can swap barrels. A longer 32 or 34 inch bbl for clays and a 28 inch bbl for the field. Recoil seems to be softer in semi autos too.
12ga vs 20ga. 20ga will be a handicap shooting clays. If you are a good shot, you will certainly break some clays with a 20ga, but you will break more with a 12ga. You will also get more pheasant.
Lastly, My philosophy about my limited budget is that I can't afford to buy junk, so I buy used premium stuff that some rich dude paid out the ass for, and got bored with.
I don't know how much of a hurry you are in to buy an OU, but if we happen to be at a shotgun shoot at the same time, you are more than welcome to try my Citori 12GA.