Refinance or pay down the principle?

5 months ago we bought a house for $480k and borrowed $280k at 5.0% apy for 30 years. we are making bi-weekly payments instead of monthly payments and can afford a higher house payment of at least $200 more per month. we now have another $100k that we want to apply to the loan. should we 1) make a $100k payment toward the principle of our current loan? or 2) refinance and get a loan for only $180k instead of the current $280k?
we plan to stay in the house 15 years, but could possible move in 3 years.

thanks for the help.
logan