As someone who finished A4 today, I would hope my post isn't too biased.
A4 has going for it a continuity of the Avernum storyline, past with our classic Exile games have to offer us. I really enjoyed it, as the storyline was fun. I think I maximized almost all of the sidequests and goodies in a single run through it and had a lot of fun doing so (save at least 13 fine leathers. You'll thank me later).
Blades of Exile is a little different. It has the definate(I know I spelled that wrong) non-continuity going for it, but with that has differnt people (queue Stareye[sp?]) going with challenging, high level scenarios that really make you think how to defeat different situations. They aren't always easy, contrary to A4s (we're going to give you stuff and you have to figure it out) its more of the "there is unknown and you have to go from there" in some scenarios.
I really think it is unfair to compare the two, since they are both made to supplant two distinct interests. BoA is a remake of BoE, with an editor and what not (I'll never forget running around with a lvl 45 part in BoE demolishing things). A4 is a continuation of the Avernum/Exile storyline. I had alot of fun with both, and have spent the better part of my winter break on A4.
I'd like to apologize if this seems to be a long winded post, but its my honest opinion. Comparing the two games straight across is really unfair as there is no real yes or no on either two games.