The enemy team is doing Baron. Both teams are equally strong, but you see a opportunity to get a inhibitor in exchange for a Baron. What is the best course of action?
To be quite honest, there are WAY too many variables at work here to be able to give you a straight answer.
The simple/quick answer? It depends.
A few variables for example: Gold lead, Positioning, Lane push, Character lineup, among tens of others of things.
If there was always an easy answer, there would be far less "comebacks" than you see already. In some cases, if you feel like your team can take an inhib safely AND defend/turtle during the duration of baron, it could work.
PERSONALLY, I feel like giving baron is far too much. Even in exchange for an inhib, Baron is game ending. If a team is any good, they'll maximize their advantage of Baron and either force a teamfight (which if they win will result in you losing mutliple inhibs) or just split push you and take turrets/inhibs that way. Again this is all hypothetical, if you could give me a more specific scenario, I could help you out :)
Inhibitor will usually be worth more than a Baron. But if you wait to initiate while they are fighting baron there is a chance for a baron steal + killing stragglers. If your team comp helps to get a jump on the enemy team it might be beneficial to fight at baron.. but most of the time an inhibitor will be worth more as you can still defend your base afterward and wait out the baron.
I'm going to have to say it depends on time of the current game. Early/mid game it is normally worth it to take out an inhibitor due to it forces the team back to defend that lane and they don't have the pushing power to retaliate on your base inhibitors while baron is still in effect (assuming teams are near equal). However, late game letting the enemy team get baron can often prove fatal due to the pushing power and finished builds can easily cause the lose of the game as turrets can be tanked and they will have a large team fight advantage.
Fight them because if they get the baron buff they can probably win a team fight if they are smart on positioning baron buff is one the things u need to have control of in order to win so in the long run baron>inhibitor if u are certain u can win a baron fight or steal a baron do it u can always get an inhibitor after the fight