I'm split between a Tubus Logo and a Tubus Cargo for a rear rack.

I have a trunk bag already, that I use for carrying a rain shell and some snacks when I'm riding with my club. I'm planning to start touring this summer, and I really liked the lower rails on the Logo, but I'm not sure that I could still fit a trunk bag on it when I'm not touring.

In light of that, I have a few questions for you guys:

1. Is it possible to fit a trunk bag on the Tubus Logo?
   
2. How much of an advantage do the lower rails offer, handling-wise? (I have a Surly LHT, so heel clearance shouldn't be an issue)
   
3. What other advice would you offer?

Thanks in advance!