The 80s have more bass by themselves, yes, but not nearly the same as what a subwoofer would provide. Depending on what type of music you listen to most, the 80s might be completely adequate without a sub. However, if you're going to be listening to movies as well, then the 80s will never go as low as a good sub would.
Personally, given the choices you laid out, I'd choose M60s + a good sub over the M80s alone (I have heard both). However, if I wasn't confined to those choices, I'd get the M80s + two or more good subs.
