I am usually
completely "in" with the school of thought that fixing something RIGHT the first time means less up-keep and repair/replacement down the road.
BUT, we only intend to be at this location 2-4 more years, and not only does this workshop need a new roof, but the house needs replacement shingles or steel roofing, also.
If I had the money, I would absolutely do them both in a good quality, matching rib-style metal and be done with it. BUT, in this housing market, I can't foresee recouping the cost if I sell in our 2-4 yr timeframe.
I am leaning towards doing a "satisfactory" job with excellent craftsmanship (BF is very good, professional remodeller/carpenter) so that the listing of this property for sale can say "new roof on home and workshop in 2010".
Now, things might change.

We are doing the shed roof demo, truss re-building, sheathing, and 15lb felt today for a dry-in. Then, depending on our income for the next 2 weeks, we will decide what final product to use on it....
Same thing will probably happen with the house, lol. It will just depend on the $$ situation when the time comes to do it.
Am I underestimating the potential value to a future buyer of a metal roof vs. a shingle roof (on the house)?