Normally a 4 ohm is the magic number for a sub. With amps the lower the ohm normally the higher the watts.
EX:
JBL BP1200.1
6oo watts RMS @ 4 OHM
1200 watts RMS @ 2 OHM
With DVC a single 4 ohm sub can be wired down to 2 ohm with wiring parrelel but it can also be wired to 8 ohm by wiring it in series. The ohm all depends on how much power you want going to your sub via and amp.\
The difference is that a 8 ohm sub will not allow as much power as an 1 ohm is. Like I said earlyer it all depends on how much power you want going to your sub that normally deturmens what ohm sub you want.
Hope this helps...
Jon