Property A without question. While I don’t like being next to a busy highway, it might make it more accessible for vets and farriers. I would not take on a project property that needed all the work done of clearing and building, there is so much more that goes into it along with the additional cost. It makes property B far more expensive than property A without adding value to property B once you’ve invested all that into it.
Clearing land, building barns and arenas is really costly, takes time and adds a lot of stress to your life. Those things also do not always add value to a property for future sales. Many of those costs are cash only options also. You will also need to do grading for barns and arenas which can add a lot of extra cost, drainage, adding water, plumbing, electricity etc. all add dollars to the project.