One mil tape takes a bit less bias than 1.5 mil, which has some effect on how hard you can push the recording levels on the tape. In my experience, it's not enough to fuss over. I'm a fan of ATR tapes, the 1.0 mil MDS is the thinner version of ATR Master; both are superb, if pricey. I've calibrated my B77 to the 1.0 mil MDS and haven't looked back. It's got more headroom than I ever dare to use.