as you have a 20D you have to multiply by 1.6 so its 38.4-168.  versus
38.4-112.  You are quite correct that 112 is more than adequate for most if
not all portrait situtations.  At this point i would also consider the
17-85IS and the new 17-55.

With an effective wideangle starting from 27.2 Your just inside landscape
territory or great family group shots without actual distortion.

As for a 2.8 zoom the 20D autofocus does benefit from a zoom this wide in
very low light.  On the other hand i think the 24-105 is IS so you can do
more handheld without resorting to tripod.  Weight and size are factors as
well as overall price.  If you are really intent on portrait work then super
sharp lenses aren't necessary.  Slightly soft focus is preferrable.  As is
shallow depth of field.  Here the 2.8 lens will come into its own.  I take
it the 2.8 is the more costly?

I don't think I'd say that most people prefer one or the other... users
pretty universally seem to love both, and any preference seems to be
just that - personal preference - and not a reflection of one's
superiority.

Shooting inside a church, I'd say grab the 24-70, because the 2.8 will
help (and IS might or might not help... remember, it might grab you
extra stops of shutter, but if you slow that shutter down, you need
nonmoving subjects). But I also have a preference for fast lenses and I
don't personally use IS as much as some. I've extensively used and
loved that lens. So like I say, a matter of personal preference.

Think about which is more important to you: a faster lens also capable
of shallower depth of field, or a slower lens with more range and IS.
Both options have merit. Both lenses are top-quality glass with scores
of supporters. Which is right for you?