I'm (still) looking at getting a 9 speed mountain rear derailleur to use on my 10 speed road setup (see here). The reason being that I want a wider range cassette, maybe an 11-32 or 11-34. Since I'm using this with a compact double I naturally thought that it would make sense to get the medium cage version (GS) instead of the long cage (SGS). I've read the long cage is for triples and the medium cage is for doubles. However, I noticed that the total capacity is:

• GS: 33T
• SGS: 45T

Now, for a compact crankset the difference is 50-34=16. Then if we add that to the differences in the cassettes listed above we get:

• 11-34 = 16 + (34-11) = 39T
• 11-32 = 16 + (32-11) = 37T

Both of these would seem to indicate I should get the long cage. But I seem to recall some saying the total capacity doesn't matter much. Maybe it means I shouldn't cross chain, which isn't a good idea anyway, but sometimes happens on accident and wouldn't want it destroying things. It also surprises me that this is the case with the medium cage mtn derailleur, is that because in those cases the difference on the front chainrings isn't usually as large as with a compact crankset?