Sugarbrie's first paragraph makes an important about quality. Cheap subwoofers are worse than none, as a rule, since they tend to degrade the sound from the main speakers.
When it comes to subwoofer design, however, one large powerful subwoofer isn't necessarily the best approach. Subwoofers with several smaller drivers (in the 8-10" range) have the virtue of being able to move a lot of air, while the smaller drivers have less mass and thus can respond more quickly than a large driver in the 15-18" range. Multiple drivers in a subwoofer (assuming it is well designed) also offer the advantage of averaging out the resonance points, thereby yielding a smoother frequency response (Vandersteen subwoofers, for example, use three 8" drivers).
The primary advantage of using a stereo pair of subs is that they load the room more evenly, thereby reducing the resonant nodes in the room. A stereo pair of subs also presents a much more realistic sonic image, since deep frequencies may be coming more from one speaker than the other. (When I bought my first Vandersteen subwoofer, I was very pleased with it, and bought the second one expecting it to just add a bit more "oomph" to the deep bass. What I experienced was a MUCH better overall sound reproduction that provided spatial clues.)
Assuming your budget allows you to buy a good quality of stereo subs, I would personally recommend that approach to having a somewhat more expensive single sub.
When it comes to subwoofer design, however, one large powerful subwoofer isn't necessarily the best approach. Subwoofers with several smaller drivers (in the 8-10" range) have the virtue of being able to move a lot of air, while the smaller drivers have less mass and thus can respond more quickly than a large driver in the 15-18" range. Multiple drivers in a subwoofer (assuming it is well designed) also offer the advantage of averaging out the resonance points, thereby yielding a smoother frequency response (Vandersteen subwoofers, for example, use three 8" drivers).
The primary advantage of using a stereo pair of subs is that they load the room more evenly, thereby reducing the resonant nodes in the room. A stereo pair of subs also presents a much more realistic sonic image, since deep frequencies may be coming more from one speaker than the other. (When I bought my first Vandersteen subwoofer, I was very pleased with it, and bought the second one expecting it to just add a bit more "oomph" to the deep bass. What I experienced was a MUCH better overall sound reproduction that provided spatial clues.)
Assuming your budget allows you to buy a good quality of stereo subs, I would personally recommend that approach to having a somewhat more expensive single sub.