Go with the prefab and cover over the window or remove it.
Tenants rarely if ever maintain anything in a rental. Grout will end up with mold if you tile no matter what you do to try and stop it.
You're right not to rely on the tenant to help maintain grout and caulking. I would base my decision in this case off of how likely I am to sell the property within the next few years. If you plan on keeping it as a long term investment, it may make more sense just to go with the pre-fab. It's less work and it likely will not affect the rental rate. If there's a good chance you'll sell soon I'd go with the tile.
Hope this helps.