Chances are, the salesman said the Dayton was low end because he has never heard of it. I am a saleman myself, but do not revert to that kind of rubbish tactic. The Dayton is a quality driver and set up, as is the B&W. I would be willing to bet, without hearing them both, that they actually have similar performance capabilities.
As far as that salesman is concerned, I do not know what he is talking about as far as resistance goes. It sounds to me like he was trying to say the sub is accurate with very little or no overhang. From what I have read on the Dayton, it would also be good in that department.
Honestly, if you are looking at a 10" near $500, I would do an SVS. with a 45 day money back guarantee, you can use it in your house to see if you really like it. For your room size, I would suggest two though. As with either of the other subs mentioned.
See here:
http://www.svsubwoofers.com/subs_pb10_isd.htm It is $429
If you do not mind a cylinder, this is available for $549:
http://www.svsubwoofers.com/subs_pci_25-31.htm
Honestly, it should out do both of them (Dayton or B&W).