Both NHT and SVS make very good low priced subs. All other things being equal (power, woofer size, etc) ported subs will play lower and louder and are considered by most to be better for home theater, sealed subs are smaller and less boomy and are considered by most to be better for music.
If you want very smooth, balanced, loud bass the tradeoff is to get two (possibly smaller) subs - this wiLl increase your placement options and smooth out your bass.
If you buy an SVS sub, I would recommend buying it directly from them - their return policy and "buyer bill of rights" are very good benefits and the prices are the same as other sellers.
2-Ch (HT L/R): Oppo BDP-105 BD, Adcom GFP-750 pre, Bryston 10B Sub Xover, Bryston 4BSST2 / Paradigm Signature S4 v.2 (L/R), (2) SVS SB12-NSD (Subs)
Home Theater: Bryston 4BSST2 amp / Paradigm CC-590 (C), Outlaw 7700 amp / (4) Def Tech UIW-RSSII (LS/RS/LB/RB), Samsung 46” 3D LCD