The image of your quality telescope will be only as good as your 5x Barlow. Get the Powermate, you won't regret it. As to which one others may chime in with a recommendation. Either 4x or 5x but note the 5x can amplify up to 7.7x depending on distance to sensor. 2x or 2.5x is probably not enough.
See
http://www.televue.com/engine/TV3b_p...d=123&Tab=_app
Note if you go the DSLR path you need to get the T ring adaptor for the Poermate. There are three, one for 4x, one for 2x and one for 2.5 & 5x. This places the sensor at the correct distance for the desired amplification factor.
If I had an NP127 I would only consider a Powermate.