get a peice of oak and try the spar and the urathane and the polyurethane and make sure the finish is what you desire. All of these leave th wood with a slight yellowish coating or look.
Also, you want whatever you get to have a good quality UV protectant. Since most doors that are wood usually take the brunt of the damage on the bottom one third of the door. This is sun, and bouncing up of rain off the stoop.
I would get a nice quality brass kick plate and install it immediately. Even lay on a ligh coat of caulk all around the inside of the plate before you screw it in to keep water from underneath the plate.
Consider this also, water based Polyurethane with uv protectant will not yellow and you can put on four coats to one of oil. This is the route I would go.
As all mentioned before, if you do this and never maintain the door after this, it will fall apart. I would designate each Spring to lightly sand the door, steel wool it and then re apply this again for that year.
You spend big bucks on this door and it deserves your attention.