both good subs. w6 more sq, w3 slightly worse sq but can do v.good spl - seen a van with 5 15W3 pushing 168dB with over 750RMS to each sub - they can take lots of power and are way cheaper than the w6s (you are talking about the old w6s arent u?)
If u are talking about w6V.2, then i havent heard one but looking at the specs and picts, it seems quite good, like a small w7. But I think if you want a mixture of SQ/SPL, then any of these in the right enclosure will kill at least 2 W3's or W6s and u can quote me on this...............
1. DD 9012/9512
2. Beyond Audio Inhuman
3. IDMax
4. HSL eXcursion pro - one of the most underrated subs ever, which bumps off v.low wattage!!!!!!!!!!!!!!!!!!!!!!!!
5. Eclipse Ti (v.expensive though!!!!!!!!! Apparently they can take around 3000RMS all day long, according to an Eclipse technician who posted on another site a while ago).
Hope this helps.