Eh? I don't see how material would affect the fit, your measurements relative to the jacket measurements affect the fit.
To your point, lamb tends to drape better since it's softer and molds better to your body than goat would.
Just AA 50/50 poly cotton T...get them wholesale...
Not sure if goat is thinner, it might be, but it's definitely lighter than lamb.
I've had lamb and goat TOJ jackets and I find the goat to drape very well + no bunching up on the shoulders. My GF's black lamb DR doesn't bunch up either but my 2010 bomber did slightly, didn't bother me though, I always thought that it had to do more with the shape of my shoulder than anything else.
btw, the more I've wore my dark brown goat MDR the more I've grown to like the goat leather. Performs well, even in light rain, feels very soft. Hard to explain, but it also has a somewhat more "substantial" look/feel to it compared to lamb, especially if the lamb is thin (which is sometimes the case even with TOJ's black lamb). I have still two lamb orders that I'm waiting and if it wasn't already this late in my order (ordered late june) I'd probably change my A-2 order from brown lamb to brown glazed goat.