I second the vote for the 5 gallon cans for your uses. If you're only running the tractor on the weekends, it's not often you'll need more than what you can carry in a can or two. Not sure where your filler is, but I like a can with well rounded corners on the bottom so I can set the can down and lean to pour. I have more issues with my small lawn tractors than my big tractor, it works out so I can prop a leg up on the step, set the can on my leg, and lean it right into the filler. In my case also I like the "spillproof" option on my can, because I can get the can turned up and the filler tube on the can into the filler of the tractor tank before enabling the flow from the can.
Avoid galvanized steel cans for diesel storage, there can be some reactions inside the can with the galvanization that will cause it to come out in your fuel system and cause costly repairs.