Things to consider, horse career ? Not alot going on, too many people ( like you lol teasing here) willing to do it for free, It is almost always better to just pay board than to think you are going to work it off, you'll end up spending more hours working it off than you would just working a fair wage some where else. I board when I am on the boat 2 weeks on 2 weeks off, but my board only comes to 5 dollars a day, It would take me more than an hour just to drive back and forth, not worth my time,
You should see what kinda hours you can expect, (ask the boss not other workers) at the stable.
Ok now with the old fart speech, you sound like you are young and I'm going to guess not supporting yourself so don't have alot of must pay bills, looks like taking care and enjoying your horse is your primary expense because someone else is taking care of your basic expenses,
Take the stable job! You are young, enjoy it while you can, you have the rest of your life to be stuck in a job you don't like because you must work to feed yourself, and maybe others.