A few thoughts
Not to overwhelm you with information, but the best way to find out is to try them both.
Most of us agree on ideals of long belly for full floaters and shorter bellys for sinktip work. Here are a few more.
Consider a short belly line if
* you have brush-covered banks or other obstructions around
you where you'll do most of your casting.
* you typically fish small streams
* you plan to do a lot of stripping (esp fishing for Coho, searun
cutts, etc
* you're not really into the art of casting and just want the
quickest way to get your fly to the fish
* you prefer to fish a shorter lighter rod, and especially if you
plan to use it to fish larger flys and sinktips
Consider a longer belly line if
* you're into the art of casting and are looking forward to
spending some time learning the various techniques.
* have open banks / larger rivers where you plan to do most of
your fishing
* use a longer (at least 14') spey rod
* will use it for mostly floating line work
Bear in mind that none of these are absolutes. I'm sure there will be plenty of readers thinking/replying, "Hell, I use an XLT on my 7136 with deep water express tips all winter. I can cast it 130', even in a headwind!!!"

To those exceptional casters, I tip my hat. These suggestions are what I've gathered from interacting with a lot of average speycasters and what they find works for them.
Additionally, there's tremendous skill in the casting of shorter belly lines. Guys like Goran, Ed Ward, Dec Hogan, and Harry Lemire show that there's a lot to learn to truely cast these lines well. It's just that these lines will also allow the beginning speycaster to cast fishable distances (60'+) quicker and more consistently than the longer belly lines. (IMHO

)
One point of dispute - Rob mentioned that long bellys are better for Extreme Long Distance. I'd have to respectfully disagree, as I've seen Goran Anderrson cast as far or further than any of the long-belly deitys with his short shooting heads. I don't know the exact conditions they cast under at the "World Speycasting Championships" where Steve, Way, and others compete. Sounds like from a pier on a lake. This may give an advantage to the long-belly casters, as most seem to choose this type of line. Then again, all the grass-based competitions (almost all the recent ones in the PNW) have been won by short-belly casters. On the river, tho, I don't think the long-belly lines have any advantage of the shooting heads when similar length rods and line weights are used.
Good Luck!
Brian