They are both good speakers, but my preference would clearly be for the NHT SB3. The SB3 is a warmer, more musical sounding speaker than the 601. I attribute this to the NHT being an acoustic suspension (sealed box) design, as opposed to the tuned port design of the B&W. I have found that acoustic suspension designs make for tighter, more natural sounding bass. The NHT's go fairly low in bass region. I own a pair of the NHT SB2's that I have paired with a NAD C320 BEE amplifier (and a Music Hall MMF CD25 CD player) and the combination is stunning ... easy, effortless, and highly enjoyable sound. The NHT's also sound good when played at low volumes.
The B&W is slightly more detailed in the treble region than the NHT. Of the moderately priced B&W line, I feel that the 600 series is the best of the bunch. If you like how the "British" sound (neutral midrange, tipped up treble), you will like the B&W's. The B&W's also have a higher sensitivity than the NHT's... so they will play louder with less power.
Regards, , Rich