If clay targets are the only use, and not on a competitive basis, either will work fine as the typical payload in either runs at 1200fps consisting of either 7.5, 8, 8.5, or 9 shot.
28s have a sort of coolness factor because most folks shoot a 20 due to cheaper ammo costs.
In Sporting Clays, the 28 has become the niche bore for subgauge events
With a $1000 budget, you are really stretching the ability to own a good NEW O/U that will be reliable and last. Used opens up several more avenues; otherwise a good semi can be had.
You might want to check out CDNN's latest catalog for discontinued guns from Browning and Lanber; also look at Guns Unlimited out of Omaha for SKB.
Target versions tend to cost more than field guns and are also heavier - a good thing when shooting a lot of targets