If you're OK with DVD resolution then grab em. There's little difference for DS9 as it was only filmed for HDTV and on 4:3 digital tape. For TNG they remastered the whole series from the film originals and it's available on 1080p Bluray for about a hundred bucks more than the DVDs. Personally i think it's worth it for the increase in quality but it's all personal taste.
I think I always knew this ... your comment just made me realize it more
I'm on Enterprise (ENT) now after just completing DS9 and years ago watching TNG, VOY and more recent years DIS, SNW, PIC and taking my time with LOW
I've resigned myself to watching Trek for the rest of my life. Neat thing about getting old is that you lose your memory so in a few years, I'll be enjoying these shows again like I did the first time I watched them.