I have also been doing some research on adding an Aux. Battery.

I started out being fairly confused.  I read some of the
notes about installing a 2nd battery

 http://www.vanagon.com/info/faq/#electrical
  and
 http://www.vanagon.com/beethoven/electrical.html

Both talked about how the space was too small, but yet the VW parts
guy and a couple of people on the list sent me notes saying that
any Group 41 battery would fit and sure enough I took out my
main battery today and it fit fine in the Aux. spot.

After talking to a few more people, I found where I was confused.
The net articles had clearly said "Deep Cycle" but since that
did not mean anything to me, it went in one ear and out the other.
(or whatever the equivalent action is with reading computer
screens)

Perhaps someone could post a good technical description of why
a "deep cycle battery" is the best battery for an ax battery.
My limited understanding is that it is designed for applications
where it is drained completely and then recharged compared to
an auto battery that is designed to be constantly charged.

So my questions are now:
  - Is a small deep cycle wheelchair battery really better than
    a top of the line Group 41 battery if one is normally not
    going to empty the battery.  (we just want to have some
    extra security that we are not going to drain the main battery
    and be stranded.  And having a 2nd battery for a jump start
    seems like a good investment.  I am pretty good at leaving
    my headlights on.

  - What bad happens when you use a std battery instead of a
    deep cycle battery.  Does it just wear out sooner?
    One person told me that the deep cycle will recharge
    faster.  Is that true?

  - Is an Optima at $130 + really worth it compared to
    one of the above alternative (which seem to run about
    $55-70 at Sears)