Quality is never a bad thing. I have resisted the Blu-Ray technology so far, but now that players are slowly reaching affordable prices, I might get one soon enough. And this new ST release is just the excuse I need, as it has both versions on it. And there is an immense difference between seeing the show (or any other film, for that matter) in HD as opposed to SD. You don't want to go back once you've seen the level of detail and sharpness, the colors etc that HD offers.
DVD remains good enough, and I'm keeping the Star Trek DVDs I have, but any new purchase would be Blu-Ray.