I've got both the BBC and NPR radioplays, and have sat down to compare them side-by-side.
On the one hand, the quality of voice acting in the BBC version is clearly superior. And more of the story from the books makes it in, as it's several hours longer. I think it's the better version overall, though there are places in the story where I think the NPR did a better job.
It is, however, a bit unfair to compare an adaptation of 13 hours (for the BBC) to 8-9 hours (for the films). And frankly, the radioplay could afford to appeal to a tiny audience of hardcore geeks (since it cost very little to make, and the BBC isn't really held accountable if they produce something that's notpopular) - the filmmakers don't have that luxury.