For the right deal, I'd trade either.
Something that has been missed by a lot of people about the team is how terrible the defense was last year. The problem is if we trade Aybar, we lose offense and depth by replacing him with Romine. It would be difficult to find a suitable replacement at SS on the market, or even with a trade (we need to stock the system, not deplete it).
Howie would be easier to trade. But, again there is a drop off in production between Howie and Green-especially defensively. I would do it if we could get someone like Bulter in return. Then I think we take the remaining money to sign a pitcher, and give a bunch of low-risk pitchers invites to spring training for more depth.