I like the shorter Skagit heads. They handle tips and big flys with more authority and cast great. But the flights and compacts are pretty darn nice. You could easily go either way, but I prefer shorts for skagit casting.
I think it really depends on your casting stroke. My personal casting stroke favors the skagit switch. I've used it on 12'9" and an 11'9" rods on love it on both. Especially for throwing long sink tips with big flies in tight quarters...awesome! If you're one of those people who like the longer lines and like that big D loop then you'll probably end up blowing a lot of anchors and not being very happy. For me with the skagit switch lines it's more of a mellow little D loop and a nice crisp forward stroke with a bunch of bottom hand and...Boom! There it goes. I am very happy.
Another option is the Scientific Anglers Skagit Extreme which is shorter than my compact skagits.
I have the 560 and it is only 23 feet (as is the 520). Use it on 11'9" inch and a 12' 7 weights I have. The 480 and 420 are only 22 feet
The switch is so money. I have one on my 7126 tcx and I personally can't imagine a better head for sink tip slangin. Like Matt said back off the gas, just enough to keep your rod loaded during the sweep. A little bottom hand, easy effortless forward stroke and flock of white doves erupt into the sky as your sink tip and fly sails across the river. Wish I was fishin!
I use a 540 Skagit Switch on my 8124 (the predecessor to the 7126 tcx) and there isn't much I can't throw with it. I especially like the Skagit Switch when I need to throw really heavy tips for digging deep on short swings as well as super tight quarters where I have to keep the majority of my DLoop in front of me.
The First rule of the day with any Skagit head but is especially true of the super short, super massive Skagit heads is slow as you go through out the cast.
The Second rule of the day is keep constant motion through out the cast which keeps a steady load on the rod.
The Third rule of the day is apply steady bottom hand power on the forward cast which will create the most line speed with out blowing the anchor during the forward cast.