Actually the pattern that any given shotgun throws is more dictacted by the load and choke than by the length of the barrel.

In other words, you could take a 26" barrel with changable chokes in it and get all kinds of different patterns out of it. Even simply by changing shot size and shot load will make the pattern vary.

The length of YOUR shotgun should be dictated more by its planned use. In other words, many guys like a 30" double barrel for shooting sporting clays because the extra weight induces good follow through on the shot. And then there are guys that like a 26" chrome lined double barrel for pheasant and quail hunting because it's much lighter to carry all day and is better suited for snap shooting quail in tight spots.

IMO, flip a coin and let it decide for ya. Just make sure that you get abarrel with change-able choke tubes and you'll be fine.