Doing a second semi-playthrough-who to romance?

Depends on what you like out of your romance and whether it's important to you that your LI follows you into the field.

Bull has good battle synergy with a sword-and-board tank or a DW rogue. Cullen has a tiny bit of special dialogue for elves, but nothing for non-mages. Cullen's got a more traditional, classically wholesome romance, whereas Bull starts out with friendly stress relief and evolves into something deep and devoted. Both have potential for major angst if you make certain choices.