I went through this personal debate myself.  I have been a VW nut for many
years and deep in my heart I wished to have an all-german van.  In the end I
used my brain instead.
I chose the Subaru for several reasons.  Bear in mind that these are my
opinions and I am sure that there will be many folks who disagree.
1.  The engine is what VW should have built in the first place.  A fully
balanced opposed engine that is quiet and smoooooth.  Many owners have
reported the fine experience of not knowing if their van is still running at
idle.  Another phenomenon is the ability to hold quiet conversations with
passengers who are in the rear seat.
2.  The KEP kit is very nice and requires minimal modifications.
3.  I chose to have someone else to the conversion for me and I found a
place that has done over a dozen conversions and the customers to whom I
spoke were delighted.
4.  The inline engines are buzzy.
5.  The Subaru has more STOCK hp and torque.  I knew that I could hotrod an
inline but it was my goal to have a reliable and low-maintenance van.
6.  The support community is fantastic for these conversions.  The links
that Warren supplied will show you that.
7.  Everything fits under the lid with no cutting.  The outfit doing my
conversion uses factory hoses that make the installation look stock.
8.  The engines are inexpensive.  You could buy a couple used Subaru engines
for the price a single quality rebuild of a WBX.
9.  A peak at this address
http://www.cycoactive.com/urabus/urabus_registry.html will show you just a
few of the owners and their opinions.
10.  The KEP kit is completely CARB approved so there are no emissions
hassles.
There is more but I do not want to waste more bandwidth.
I know that there are many stories of WBX engines going multiple thousands
of miles with no problems but these are usually restricted to original
owners with exceptional maintenance habits.  Since most of us are buying
these vans used, we do not have the luxury of immaculate maintenance.  It is
much easier and less expensive to find a good used '90-'94 2.2 Subaru engine
than it is to rebuild an 80's WBX.
My opinion for what its worth,
Mike Pidcoe