It makes more sense to trade for Howard than trade for Horford, though I like the latter player much, much more.
Howard is at least under the Cs control for another year. He fits a clear need as a dominant, huge, rim protecting big who can rebound and dunk a lot. We simply don't have that on our roster, anywhere. I am not advocating we trade for him, because we shouldn't, but he fits a need for sure.
Horford is like a better version of Amir Johnson. He gives you more on the offensive end for sure, and he would make the starting 5 much better. Does he make this team an NBA finals contender? Maybe, but that the Cs with Horford beat the Cavs if the Cavs play well. There is no doubt in my mind that he fits the Cs well and complements the current roster.
Having said that, there is zero guarantee he resigns and quite frankly if I was him I wouldn't sign an extension with Boston if I was traded. So if you are Boston are you willing to trade what it will take to pry Horford away from ATL (most likely at least one good young player and a 1st round pick) just to maybe get a slight chance at beating the Cavs/Raps this year to get to the Finals? I say NO WAY.