I voted Violence but I don't really think you can compare the two. No genre is better than another, you can like one more than another though so the topic is fair.
Even so, I appreciate both action manga and romance. My favourite manga of all time is Fruits Basket, a Comedy Romance, and yet in terms of genres I much prefer reading action manga/violence. I don't think I really can say I like one more than another. Both can easily be done badly. Romance becomes too cliché or Violence becomes too action-packed and loses any form of story. When they're done well they're usually brilliant.