Blu-ray is always going to be better than DVD. Enterprise was filmed in High Definition obviously and if you get the DVDs, they're not going to be in HD. I guess it's up to your visual standards whether or not the difference is worth it.
Sorry, but I don't agree. Some older series and movies are simply upscaled to appear HD, which can sometimes result in pretty lousy quality. My br-player is set to play everything in Native settings, meaning a dvd is not upscaled. I've seen the effect of upscaling, and sometimes it's just horrible.