OK I'll offer my $0.02.  Have not heard the complete output (!) of either.
Have seen the "old" Celi on video (Bruckner, Prokofiev).  Also Karajan
clips, have listened to as much of him as I care to, but will always be
on the lookout for a good K. recording.  The St. Matthew Passion is NOT
one.  Judging from what I've heard of the opening chorus it's pretty
nasty.  I like Celi's passion for perfection over K's interest in turning
everything to mush (especially in later years--or am I being influenced
by conventional wisdom?) but neither is a particular favorite conductor.
There are a couple of K opera recordings that may be pretty good.  I can
enjoy his Strauss "Vier Laetzte Gesaenge" with Janowitz.  But the image
of him as posturing superstar on an ego trip stays with me.  My favorite
conductors are Furtwaengler, Reiner, Barbirolli, Bernstein, Klemperer,
Walter, Munch (at least in Berlioz), and Toscanini (in certain repertoire).
Of course I like certain other conductors in favorite recordings--Boult
for the Planets, for instance.  So the bottom line is that I only like
conductors for what they do, how they deliver the musical goods.
Consistency is important to the extent that one somewhat knows what to
expect when buying an untried recording.  But one should keep one's eyes
and ears open for surprises.  I'm not good at that and must rely on word
of mouth.

Chris Bonds