Long cage or short cage derailleur?

[cbPDX]

Building up a cross frame for the new season and was wondering what people think about running a short cage or a long cage derailleur.

I like having a range of gears. I am more of a spinner than a masher of gears so I will be running a double 36/46 up front. Have a 12-26 cassette for the back but wouldn't mind trying to sneak in a 11-30 mountain cassette.

I currently have a couple of long cage derailleurs (105 & Ultegra) in the parts box. (I am running a 105 front derailleur).

Guess some questions are:

What is the largest sprocket someone has been able to get away with using a short cage derailleur? Not the rating, but real life. (I know the "capacity" and "largest sprocket" ratings are usually pretty conservative.)

Advantages/Disadvantages with short cage/long cage on a cross bike?

Depending on answer, anyone want to trade a new 9-speed long cage ultegra for a short cage? :)

Thanks

[slagjumper]

I would stick with Shimano's specs. My gut is telling me that you could use the short. Can you post the S9 max tooth, min tooth and capacity specs for the 105? I only see the S10 specs on the website.

[Iffacus]

I know of a few people that have managed to get way with a 27 sprocket on the back with a 105 shortcage rear mech.