The 3M adhesive is easily dissolved by REAL brakleen(not the cheap no name crap), or permatex #4MA gasket remover. I do agree, however, that it can make removal of the gasket a bit of a bear. On v/c, I use about five (small) spots of adhesive, so if I need back in there in the future, I am only scraping the bare minimum. Ask any mechanic, we all hate scraping.
BTW, my vote is for aluminium, as when used with a good quality gasket and tightend properly, they rarely leak.