AFAIK, although I own only one BlueLine steamer (more about it in a second...), there is only one difference....the BlueLine locomotives have no motive control except what your DC transformer allows you. That is, there is no DCC decoder or component of one that controls the motor. It is only a DC sound decoder. Period.
The Paragon 2 series, of which I have several, are really very good, on par with the quality BLI managed early in their marketing career ten years ago. They have a single decoder, but it is DCC and sound. Therefore, it has comprehensive motor control, and sound, but also smoke.
The only BlueLine steamer I have is a refurbished A Class 2-6-6-4 from the Norfolk & Western. I had my buddy install a QSI Titan in it. ZOWEE!!! That is one suh-weeet piece of hardware. Both the decoder and engine are top drawer in my estimation.