Should porch roof pitch always match house roof pitch?
I think a lower pitch would look fine, and is in fact seems to be a common design choice for a porch. I think the grey metal roof would work better on a low slope roof than asphalt shingles, but won't be cheap.

We have a 3-in-12 pitch roof and have used good quality asphalt shingles. Perhaps I didn't demand the proper underlayment treatment and our house does have inferior roof decking at 3/8" thick plywood, but we have had problems with leaks.  The slightest dip or discontinuity in the roof decking has led to leaks.