The movie is good, though it will likely bore today's average movie-goer to tears. Smart dialogue, very well acted, no action whatsoever; you watch it for the pleasure of seeing two great actors at work. It may not be quite as good as The Lion in Winter (Widescreen)
but it's definitely worth watching.
The Blu-ray version is a rather stingy production. The video transfer is below the quality of a present day DVD release. There is noticeable flicker, and powder-like texture in some scenes. The Bonus features are almost non-existent, and although I haven't seen the DVD version of this, I expect that there is virtually no difference between the two.
Based on that, I would have suggested you opt for the cheaper DVD, but it turns out that they're both priced the same. If you're trying to get your kids to watch it (good luck with that) then you'll likely have to trick them with the "hey, look I got one of them cool Blu-ray movies" acts; not sure it will hold their attention for 2 and a half hours, but a bit of history will stick to them, and maybe it will pique their interest enough to pick up a book, or at least Wikipedia this Becket dude.
UPDATE: the price of the DVD has decreased on this website, making my comment/argument in the paragraph above a double negatory (huh?).