I don't think there is a 'better' or 'worse' accent, they're just different. Even within both countries there are different accents. I can say there are perceptions or stereotypes for the different accents though. A lot of Americans think the English accent is formal, intelligent and attactive (it's exotic yet I understand it!). American accents are sometimes seen as very casual. I would recommend asking two questions 1.) Do you plan on visiting either United States or United Kingdom? and 2.) What's the most appealing to your ear? Then go with with it.