I suspect the bluray stuff is hype, when we're talking about releasing old shows that came out in the SD days. Aren't old TV shows just upscaled to HD with fancy filters? That's not the same as true HD content. But if you are a completionist fan of ST, bluray is for you. I'm sticking to watching it on Netflix for now. You don't have to fiddle with discs this way, and any episode is a few clicks away.
Star Trek was shot on film which still has superior resolution to the current Blu-Ray and the forth-coming 4K. The film is scanned frame-by-frame so it is not at all upscaled; in fact its down-scaled for Blu-ray since the film resolution is far superior. Even so the resolution is so far superior to DVD; that for me it's a no-brainer; go with Blu-ray. The show looks absolutely fabulous like you have never seen it before.
I hope that clarifies.