Here's my ice in the cider:
The plain old Criterion DVD is the best content-wise, because it includes the best extras, especially the "Withnail & Us" extra that interviews Bruce Robinson, R.E.G., Paul McGann, Ralph Brown, plus casting guru Mary Selway (R.I.P.), and various fans and semi-celebs on the making of the film and its influence in their lives. There are parts of this that make me a little weepy, especially when Paul McGann is remembering the good old days making the film.
As far as actually viewing the movie, the Blu-Ray DVD version is far and away a better viewing experience. They've brightened and sharpened the picture, so you can actually see the stuff in the background, and they've tweaked the sound so that you can turn up the volume loud enough to hear the sometimes mumbly dialogue and properly enjoy Jimi Hendrix's screaming guitar.
It's beyond me why they didn't include the extras on the Blu-Ray, since the higher cost of a Blu-Ray disk usually means you get lots of other goodies on board...
Basically, when I watch it I keep both versions on hand - I watch the movie on Blu-Ray and then watch the extras on DVD.
... followed by yet another anecdote about his sensitive crimes in a punt with a chap called Norman, who had red hair and a book of poetry stained with the butter drips from crumpets.