Many times the difference is subjective. I’ve never personally used Vegas, but I doubt that the “quality” will vary (esp if your primary source is DV), from what I’ve read on various forums, people complain about Avid’s lack of options for file type importing and also I think I’ve heard that renders are slower. Again this is all subjective.
My true advice is that if Vegas is working for you and you’re happy with it, continue using it. It’s only a tool after all. But, if you want to “peek over the fence” and see what color the grass is, find a reseller that would be willing to loan you an Avid XpressPro system for a bit.
I would recommend Avid FreeDV as a taste tester, but I feel it’s designed more to allow a user to get to know the interface and workflow in an Avid system and would not necessarily live up to your expectations given your prior experience on Vegas. To truly try and compare oranges here you’d have to get your hands on an XpressPro system.
Kenton VanNatten
Avid Editor (for hire)