DWS06 Plus is brand new for this year. The DWS06 are being sold at similar prices, I think it is to help clear them out or to have people just buy the new DWS06 Plus.Make sure to check date codes on the tire. Just to make sure they are not selling you a 5 year old tire.
The only difference I can see between them is they inverted the sipes for one channel, probably for noise reduction. One caveat is on my SHO with the factory 255/40R19 Badyear RS-As, the 275/40R19 DWS06 was slightly noisier. Nothing as bad as the BFG Comp summer tires I ran on the Mustang recently, whoah boy they sounded like swampers.
There is supposed to be a rebate campaign in June and November of this year for the DWS06 Plus.