They're both beautiful to observe depending on where one is situated.
However I tend to want to sleep in and therefore a sunset at the beach is my idea of a romantic occasion.
Having said that I was once on the island Maui Hawaii and we got up super early (3 AM) to drive to volcano Haleakala to watch the sunrise. It was one of the most magnificent things I ever witnessed. It appeared as if the sun was coming out of the crater.
You drive away feeling as though you've witnessed a sort of miracle.