The TD18 strikes me as more a competitor to the B&C 18SW115 (awfully hard driver to top) than the NS18. The B&C probably has a better suspension, based on Klippel measurements of the B&C and other AE drivers. Compared to the two of them, the NS18 has considerably more throw, a more expensive motor (lots more steel, due to the underhung design, and copper sleeving like the Lambda), lower powerhandling (single-layer coil that's smaller than the B&C's coil and shorter than both of the other two), and considerably lower efficiency. I think it's also the best looking of the lot, with the Mercury space capsule profile of its basket and motor cover, and that cool-looking metal cone with pointy dustcap. (Metal that seems to ding less easily than the Dayton or TC sub cones.)
Though in a sense all of them are equivalent in that they're 18's with good top-end extension, great motors, and all three of them would be plenty of volume displacement for someone like me. In fact, the main meaningful difference between the NS18, TD18X, and B&C 18SW115, it seems to me, is that the B&C 18SW115 is the only one for which there is anything like a reliable and continuous supply! If I were building a production supersub, that's the one of the three I would use. Though in truth I wouldn't use any of the three. I'd probably use dual-opposed Peavey LR18's over any of 'em. The LR18 is a very, very good driver, and way cheaper than anything else that good.
In all seriousness, I would be shocked
if anyone could tell any difference between them at all, after suitable EQ and limitin levels to what the one with the lowest rated Vd should reasonably be able to handle. In the last (and only) test of elite-level subwoofer drivers I did, I could not find any difference whatsoever between the Maelstrom-X Mk. I, a JBL W15GTi, and an Aura NS12-794-4A after EQ. (Levels were limited to what the smallest driver, the NS12, could be reasonably expected to handle.)
Truth be told, looks and symmetry (all of the subs in my main system were Aura NS-series, so I figured the nearfield sub should be, too) were the only
reasons I replaced the Mael-X with the NS18.