See
http://www.televue.com/engine/TV3b_p...d=53&Tab=_back for an explanation of the difference.
Visually the Powermate allows the eyepiece to perform as designed with no shift in exit pupil and therefore no vignetting. They are heavy though in the 2" sizes but do give excellent results. One of the best views of Saturn I ever had was with a 35mm Panoptic and 4x Powermate (10.1" f6.4 Newtonian) but I did need a counterbalance even at high altitude as the weight was too much.
The 2.5x Powermate was redesigned I believe as the original would not take filters. Bintel still show the original on their web site but Tele Vue show the newer type. I cannot confirm that the new one is threaded but I see no other reason to change the barrel. I have the old one. See
https://www.bintel.com.au/product/te...v=6cc98ba2045f
The Tele Vue Barlow is excellent though. If you plan on using with eyepieces that alresdy have a long eye relief such as 32mm and 40mm Plossl then the Barlow is not the best choice as it will throw that eye relief out further and for such eyepieces either a 2.5x (1.25") or 2x (2") Powermate is recommended. The 2" Powermate comes with a 2"-1.25" adapter.