I'm in the process of building up a Surly Big Dummy frameset and am planning to use a Shimano XT M772 Shadow Rear Derailleur and a Shimano XT M770 9-Speed Cassette along with a Sram Chain.

My question is the derailleur length. I have the choice of a medium or long cage and was wondering if one has an advantage over the other on a cargo bike setup?

I'd also appreciate any comments about my choice of components.

jed