I think +20 JS is actually better than 15 JS. The thing to consider here are the differences compared to defensive match-ups. For example, in this case, 15 JS against 9 OD would be very good; +20 JS against 9 OD will be a kind of better or equaly effective as the case with 15 JS. BUT, 15 JS against 16 OD is not a good deal while +20 JS against 16 OD would be very good.
This is interesting, and is pretty close to what I was thinking. However, I'm still interested in what you would prefer over these options:
-5 IS vs 4 ID
-15 IS vs 14 ID
(ie: a case where you have exactly one level difference)
Run of the Mill Canadian Manager