In order to be strong you don't necessarily need to lose speed. Muscle weight doesn't cut speed that much.
If this topic does not limit me to choose one, I'd say work at both! None is better without the other. Fast strong strikes all the way!
But yes, in most situations I'd choose strength. Rather than running away and getting haunted by things forever, facing them is the better way.
But if it was something I'd like to add to myself, then it would be speed, since I already have enough strength...