Go for flat
Like Seth and suggests, I would also go for flat.
My first workbench was glued up beech, and it was glued together without consideration to the grain orientation. I tried to plane it once, and it looked worse after than before.
I would say that the trick is to make sure your plane is set to take a fine shaving, and I would start out planing diagonally.
In old benches there are also the danger of old tacks and nails that are hidden, and they will cause a nick in your plane iron.
If you have the possibilty to remove the top, you could check some local shop if they have a wide belt sander that it could be fed through. That might not be an “all hand tool” approach, but if it gets the job done and gives you a flat top – well then I think it is worth a try.
Good luck with the project