Over the years I have used horses, various medium sized older tractors (50 to 70 hp), a new compact 4wd tractor with loader, old walk-behinds and a newer BCS tractor. I have a friend who got into oxen.
I agree with others here, that draft animals are a big commitment but it can be learned if you want to go that route. It is more sustainable of course, but the draft animals should be multi purpose to be really efficient. Horses and oxen eat a lot of hay. And of course there's a lot of manure to spread if you live in a climate with a winter. And a lot of time spent building relationships with the animals. And you need to learn about harness and keep it in shape; and buy machinery to do the work, which will also have to be maintained. Horse equipment can be harder to buy than tractor equipment.
Tractors require a commitment to keeping things in shape as well. As many have mentioned, you may need to become somewhat of a mechanic. Or have a friend that can help you out. But a tractor with a loader can do a lot of work. Even something like an old Massey 35 can do really well for you. IF you have heavy work to do, such as moving manure, rocks soil, etc, a tractor could almost be considered a neccessity. But it really depends on your plans. If the heavy work is only for a short time, you can rent or hire the equipment.
A walk-behind tractor is a really handy smaller tool, and it also gets less done than a larger tractor. Especially since you don't have a loader with those. But we really like ours for pulling a no-till seeder, flail mower for the garden, and rotary plow. To buy a new one is too expensive though, so you would want to find a used one. If you can't, you may want to look for a small old tractor. But every situtation is unique, and only you can decide which is best for you.