Are you buying blu-ray versions of older movies?

Because our DVD player happens to be close to the end of its life, I posted a question in another thread whether to replace it with a very good conventional player like Oppo or a Blu Ray one. I have learned greatly from all the replies and thank everyone who posted one.

I guess what it comes down to is this: BD format just "won". Therefore, if I am not buying Blu Ray DVD's now, it would be better to postpone the player purchase and get an Oppo instead, as BD units are expected to get better and cheaper in near future. However, if I am buying Blu Ray DVD's now, then of course we need a player to play them.

Looking at our DVD purchase requirements, what is desired in our family right now are older "Nature" type films, from PBS and BBC, as well as March of the Penguins etc. None of these were shot in HD. Therefore does it make sense to buy their BD versions or would SD ones be just as good? That will decide the player we buy for the time being...

I can see the jump in convenience if not always quality from VHS to DVD (for older material), but would you buy the BD version of the older movies or stick to SD?

All advice greatly welcome.