I wear my GTs exclusively as wet weather boots, been doing that since 09.. naturally that means I wear them days in a row if I need to. I don't know about sweat but rain has never made them so damp I've had problems.
It's the Italian calfskin in olive. Every time I need to go outside I smile when I reach for the jacket.
Re: calf v lamb, the break in period really isn't something to be feared. I've got three cow hide jackets and though you'll want to wear it a bit at home first for the creases to set in your arms, once you're good the structure and the way the jacket sits on you makes you feel like a million bucks.
Honestly I'm not the biggest fan of the MDR. I think it looks kinda neutered. Go with the 2010 with the buckles and scalloped hem. The design itself is iconic enough for it's "flashiness" isn't really an issue. You're still wearing a pretty familiar, recognized design. I would say differently if it were a stone gray julius with 28" sleeves and a holster.