Find one that fits you well (stock fit) and concentrate on the basics (Head on the stock, Eye on the rock and follow through) and the barrell length isn't going to make that much difference between 28-30. Some Trap shooters lean towards longer barrels for sure and some SC guys are leaning towards longer ..but there are alot of really good shots who prefer a 28".
Check Rollin Oswald's posts on fit..or better yet his book.
Check Rollin Oswald's posts on fit..or better yet his book.