I don't know of any site that has a comparison of screen caps, but I've got both the Complete Series DVD set and the Season 1 Blu-ray set. For the past year or so, my fiancé and I have been watching one before bed a few nights a week. (She had already owned the series DVD set, and I had the BD.)...
I'm willing to pick up the first season at the current price - I'm really curious to see how the results look. But at this price, I can't imagine picking up every single season either.