I'll give a slightly different opinion
I'm much more in favor of a ration balancer, or even a vit/min supplement, over loose minerals or mineral blocks. According to the studies I've seen, there's no scientific evidence that horses self regulate any minerals other than salt. Even if they could, they don't have the ability to eat only the mineral(s) they're missing with a loose mineral, and a mineral block doesn't have enough of anything (other than salt) for even a single day in the typical 4 lb block. They also contain NO vitamins, which are severely lacking in a mostly hay diet (versus a fresh grass diet) Feeding a RB or vit/min supplement guarantees the horse is getting a minimum amount of nutrition each day, and is balanced for "average" forage. A RB has the added advantage of protein, which includes lysine, an amino acid that's missing in a lot of equine diets.
If you had a hay analysis (or at least knew if there was anything specific that's commonly missing or overly abundant in your area) you could figure out which specific RB best fits your hay, but without that it's up to your budget, availability in your area, and personal brand preferences.
Here's a good link for learning more about RB's: Ration Balancer