I'd buy the used one with 5 mags. That's a steal, and there is very little chance anyone has put enough rounds into a CW9 to wear it out. It's a small concealment handgun. Unless somebody was the king of their local backup gun matches, I'd be surprised if the average used CW9 has more than 1000 rounds through it. I'd bet somebody bought it, bought a bunch of mags for it, then had financial difficulty and sold it for a loss.

Figure mags are $40 new, so you have $160 worth of magazines over a new one. If you sell three of them on eBay you'll make $75-90, easy. That puts your real cost of the used one down to $185.

Buy the used one. Clean it up. Shoot it. If you don't want 5 mags, you can sell the extras and make some money.