Personally, I'd rather go for Greg Monroe.
He's a similar player to Horford but younger and without the worrying injury history. Also I think we have a better shot at him.
Not similar.
Horford is miles away on the defensive side of the floor.
Not so sure about that.
Monroe is an average defensive player - not a defensive anchor, but not a liability either. Horford is above average, but I wouldn't say he's exceptional.
Look, if Horford was healthy I'd take him over Monroe for sure, but he's played a total of 139 games in the past four seasons for an overall attendance record of 39% over that stretch - that's really bad. Almost Lakers Andrew Bynum bad. You basically need to build your roster around the assumption that he's going to miss half a season.
Horford's recent injury history makes Avery Bradley look as durable as Karl Malone.
I really, really like Horford as a player but the problem is he is a REALLY high risk, and I just don't know if a healthy Al Horford is dominant to the point where he's worth that risk. He's no Anthony Davis, Dwight Howard or Demarcus Cousins - he's more of a complementary star rather than the type who's going to carry a team. At least that's how I see it.
Greg Monroe over his career has played in 309 / 328 games over his career for a 94% attendance record. That's pretty impressive. In three of his four seasons he has missed 2 or less games. He's consistent, he's dependable. He may not be quite as good as a healthy Al Horford, but he is basically 90% of Al Horford in terms of production...except you have the bonus of dependability and tenure (he's only 24 years old, so many years left I him).