I think it is a 'whole enchilada' thing. If you are spending the money
to replace anyway, it might be worth considering spending a little more
to put in added capability that you might find yourself needing sometime
in the future. In the case where you do end up needing to sell, the
relatively small delta cost to get more capability that you might or
might not use now might be worth it when it comes time to sell. Think
of it as the delta for the GPS install, not the full cost, if you are
already replacing a radio.

If I were replacing my airplane today, I also would not consider one
without an IFR GPS. On the other hand, I'm not rushing out to replace
my servicable dual VOR, dual glideslope and Loran stack with a GPS unit.
I'll strongly consider it when the time comes that my radios break and
are not economically repairable, however. If it happened today, I'd
probably hold off on a GPS upgrade to see how the next gen ATC falls
out, as I'd hate to have to change out new to me equipment to meet new
equipment requirements such as ADSB in a few years.