I think a lot depends on what you want to accomplish, and which speakers combined with what cabinet. Assuming you are comparing similar (quality, power handling, efficiency, etc.) 15" and 18" drivers, each in a cabinet designed for them, the 18's will generally have a lower frequency response curve than a 15.
If you are mostly using the 410 for your overall tone and just want to "move air" underneath to give it some depth, I would say that a single 18 in a ported cab designed for it would work the best, if you want something to extend the bottom end but still contribute some color to the 600hz-3k ish range, a 15 will generally do that more efficiently.
Either will sound cool, there is a reason that a 15 or 18 under a 410 has been so fashionable among bass players for the last 15 yrs or so.
IMO, it's all good, it truthfully depends on what your ears tell you. You should try to give a few of each a listen.
