Manali is the better option but before going on the month of may check weather.
if possible go patnitop it was such a untouched beauty and in the may end weather is also great.
For a honeymoon trip, Manali will beat Mussoorie anyday. The landscape of Manali has been set by nature in romantic tone. The mighty mountains, the never ending riverbanks, the divine trail of temples and some of the finest resorts in North India will surely add spice to your honeymoon trip.