It may just boil down to preferance , or how modern you want to go , ie making huge haylage bales etc .
But tractors will always cost , fuel . Oil , mechanic , parts , tyres , apart from the mechanic bit that you may be able to do youself , all these things will mean money you have to earn somehow .
Horses can need a vet and if you do a lot of road work a farrier , but you can grow all their feed , they can be free atm , they always start and get a mare and you can have as many new ones as you want .
Belt and braces , ie both , would be the way to go if money allows .
But horses to do what you can , more or less everything once you gear things to suit , and then get someone in to to things if need be .
Working horse gear can cost a fortune , but lasts forever if looked after .
Swings and roundabouts on somethings , but you can't talk to tractors .