If you want a good alternative multishot CO2 pistol, then the only realy (affordable) choice is the Smith & Wesson 6" or 8" revolver. It's accuracy is well proven and it has a 10 shot mag instead of the 8 shot found in most of the Umarex clones.
The barrels on the S&W are interchangeable, so you can find which suits you best (mine's the 8" nickel version). The standard sights are fully adjustable for windage and elevation and the sight blade is replaceable with different size units. However you can easily replace the open sights with either an 11mm rail or a Weaver one, then mount the red-dot sight of your choice. Hell you can even get silencers and shoulder stocks for them! (note: shoulder stock not allowed in comps). Personally I wouldn't go for the Skiff (I do have one though ), although it's a 28 shot pistol, that trigger pull is just WAY too heavy to make it practical for competitions, good for informal plinking though. Pellet jams can be a right pain in the %&£$ as well.
Most members of this BBS will attest to the quality of the S&W pistols. There are other pistols you could consider, such as the Rohm Twinmaster / Trainer range, but they're less adaptable and more expensive. The Steyr LP5 is a fun little semi-auto pistol that can rattle off 5 shots in the blink of an eye, but you'll need a second mortgage to buy one .
Hope that's of some help.