G-Dub870 said:
look for a 870 express super magnum if you might get into turkey hunting. if not, a 870 express magnum will do you just fine. you will also like the pump better than semi-auto because there just that much more to go wrong with gas systems and that junk.
I concur with oneounceload's opinion about auto loader reliability. The OP is asking about a gun for sporting clays. IMHO, as far as pumps go, an 870 Wingmaster would be a better choice for targets than an 870 Express magnum. Also, an overwhelming majority of comp sporting clays shooters would select an auto over a pump. Here's my two-cents worth on the subject:
Before WW II, the Winchester Model of 1912, or just Model 12, slide-action shotgun was the darling of Skeet shooters and anyone else who competed with a repeater. After the war, non-humpback auto-loaders, like the Remington 11-48 started replacing the pumps as the competitor's choice. Why?… because they shot better scores with the auto-loader -- it's that simple.
Some die-hards found the reciprocating barrel of the R11-48, and it's predecessors, distracting and held on to their trust old pump guns. Sixteen years later, in 1964, the Remington Model 1100 was introduced. With it's locked barrel and a gas system the auto-loader had come of age.
Many will remember that 1964 was the year Winchester did a major re-tooling (to reduce costs) and the Model 12 was replaced by the Model 1200. The Model 1400 auto loader was introduced, too. To many, 1964 was a watershed year for sporting firearms: the year when auto-loaders supplanted pumps as the stick guns of choice. Many have preferred an auto-loader, but their budget has dictated a pump.
For 40+ years pumps have continued as entry level guns. They are typically purchased because they are the cheapest repeaters available from the various manufacturers. I suspect, even the most vocal proponents of M-500s and R-870s would have selected an auto had it been the same price.