a Powermate is a Barlow with (2?) extra lenses that make it telecentric. there is no performance difference in this application, except that a Barlow can give you varying magnification whereas a Powermate is specifically designed so that it doesn't.
If interested in details, this is a good summary on Barlows and telecentric Barlows -
http://www.brayebrookobservatory.org...BarlowLens.pdf
With a 618 camera, you need a final FNo of about f28, so you want about a 4x magnification Barlow or Powermate for your scope - just like
http://www.astrobin.com/users/FranckIM06/
suggest a 3x Barlow with a short extension - that will allow you to vary the magnification to suit the seeing. If you do get a Barlow, buy a good quality one.