I have both a hard keeper and an easy keeper. I'd choose the easy keeper, myself.
You can cut back an easy keeper's feed - how much they weigh depends entirely on how many calories you allow them to consume. They might want to eat more, but you can keep them healthy (this assumes you didn't let things get out of control in the first place and end up with bad hoof issues).
But how do you get calories into a horse that just won't eat enough? Not to mention, it only gets cheaper as you cut back on the easy keeper - no grain, just vitamins. The hard keeper gets more expensive as you add ulcer treatments, digestive supplements, things to balance the gut flora, different types of hay to tempt the palate, various fats and oils, beet pulp, complete feed, etc. Sometimes the issue is solvable, but with bad teeth, poor digestive absorption, chronic gut acidity, it can take a long time to get to the bottom of things and sometimes you can't fix it.
There are horses that have issues that make them feel hungry all the time. I've read about leptin resistance (no "shut off" for appetite), and horses with Cushing's have elevated cortisol levels - if you've ever talked to someone who has taken steroids like cortisol or prednisone they can attest to how hungry that makes you feel. Still, considering the horse's health and not feeding them as much as they'd like to eat, the weight can be controlled and the insulin resistance kept manageable if that is the problem.
I believe the main obstacles to an easy keeper are if you don't have the ability to manage the environment: say you don't own the property and can't make a good setup without a lot of grazing. Or if you don't have someone who can feed the horse to your specifications.
With the hard keeper, they can be sitting up to their ears in lush hay, pellets, everything else, and still be thin!
And there's no compunction about exercising the horse that tends to be chubby - more is better. You don't have to think about losing those precious pounds by riding too far.
