I would agree for the most part, unless you're thinking of using a clip on, which is part of the "SPR" concept with the full length rail. 2x on the low end seems to be your friend there, more than 3x or 4x. Someday, once I've got suppressors for all my favorite guns, I'll be saving up for a clip on, but I imagine most of us with an "SPR" aren't rocking clip ons.
So, for 99% of people with an SPR type rifle I see few if any reasons to not go with 15-18x top end for magnification with a piggy back RDS.