I just ordered the SVS 16-46 PC-Plus (B stock, saved about $80) for floor space, bass extension, weight and price. Also, shipping is only $30 or so. It's about half the weight of the SVS box sub with similar performance, the 46" port is much more efficient in the low bass and it takes much less floor space than the PB12-Plus (450 square inches for the box, 201 for the cylinder). The ultra also looks great, but at $1400 (including shipping, compared to $900 shipped for the b-stock 16-46) I think I'll wait till I have a bigger apartment to upgrade. The box seems to be flatter above 100Hz, but if you set the crossover to 80Hz as is usually recommended, that's not much of an issue.
From the SVS website: "The closely related [to the PB12-Plus line] PC-Plus line of cylinder subs remain the price/performance leaders in this class, but if a box form factor, wood or gloss finishes are your thing, and you demand world-class performance, this is your sub."