Which adds the most resell value to a home in Manassas, VA (within
commuting distance of Washington D.C.), a wood burning fireplace or a
gas log insert?

I would say this is up to the potential buyer. As far as overall
value, I believe the traditional style fireplace holds more of an
aesthetic and romantic feel, as well as being a more natural feel.
With the gas, there may be less maintenance, but you also must be wary
of leaks.

What kind of home is this for? A wood burning fireplace can be an
expensive venture if the house does not already have a predetermined
space for it.

-Burly

The home in the Manassas area will be on the market early next year. 
The current fireplace needs to be relined due to a crack.  The owner
is trying to determine whether to reline or add a gas insert instead. 
The housing market in this area is pretty hot so we believe the house
will sell fairly quickly no matter what action is taken.

It will be a very expensive proposition to repair the crack in the
fireplace.  A ventless insert would definetly have a better return on
investment.