big size difference, the PT6 is a true 5/4 and the Neptune is a true 6/4. I tried them both and they both play well and sound well,both had good intonation and ergonomics, but I chose the PT6. I found the sound of the PT6 to be a little richer (?? whatever?) and I found that the Neptune tended to back up on me a little quicker and would start to bark. BUT, Dennis (from new world) has an amazing Neptune (which we both think might be a fluke) that plays outstanding. It feels like a big version of my 6 and just playing it made me consider trading in my horn to get a Neptune. I just don't know what the chances of getting one that plays as well as his are. Both are good horns and if it matches up with your face, mouthpiece, the way you use your air etc.. either one would work out just fine in my opinion.
Hope that helps a little,
Bill Pritchard