Hard to say which is the "best" conversion.  Each has its pro's and con's.
I-4  can be bought cheaply used and is a vw.  Mated to a Wasser trans and
it gives good power and up to 24 mpg.  If using a diesel bellhousing you
don't go above the decklid.  Con you usually have to modify the driverside
engine bay because the intake manifold hits the wall on that side.  Pro,
most VW mechanics can work on it, but you get "wow a golf engine in a
Vanagon?"
I-4 Diesel,  1.9L TD fits without much grief, gives 75hp or so and is
exempt from smog testing. It seemed hard to comeby used(when I looked) and
expensive $3000+ new.  Same with TDI, but TDI much harder to install, and I
have seen much talk of busting clutches.

Subie, fits under the decklid, good power, good economy.  Con, oil pan
hangs really low, more expensive, and most mechanics take one look and say
to go elsewhere.

Whats ideal?  Cost the I-4,  Power Subie,  Reliability? Both seem pretty
reliable if you did it right.
For me I would love a V6 TDi from a Euro Passat with a Porche tranny, 150HP
and probably 28mpg, but I don't think anybody has done it.

Mine is a 1.8L from a Golf GT, why did I choose it?  For starters the van
is a 82 Diesel, so it started life as an I-4.  Secondly my friend had a
Golf GT that got into an accident.  Total cost $1200-1300 and I get 24 mpg!
 Would I like more power, yes, but it climbs hills much easier now and goes
easily to 80 mph.  Can I live with it, yes.

Hope that helps,
Dave K.