Elder dragons takes longer, but is often less risky because it doesn't deal as much damage as nashor (unless your team is so fed to where you kill nashor in seconds). Generally I would say baron is "harder" to get in that your team is more vulnerable while doing it.
Baron buff provides a longer lasting impact on the game. The buff itself is longer, and he gives the team a hefty chunk of gold to invest in items. Elder dragon, on the other hand, lasts for a short amount of time so the team using it is highly pressured to do something with it less it gets totally wasted, as I don't think it gives any gold for killing it. Elder dragon's usefulness also varies drastically depending on if your team has been keeping up on nabbing drags throughout the game.
I would say that with no dragons baron is much better, but with 3+ drags then elder is more valuable of an objective. This is likely intended, so I am personally happy about where the epic monsters are at.