You will be able to select a seat, so that is not a concern. Seat selection might be available on united.com, but it will definitely be available of Lufthansa's website, and if you do book with United, you will get a record locator number to use on the Lufthansa website.

Personally speaking, I would probably book this through UA, given their more flexible change policy, and the fact that reaching UA in the event of a schedule change will probably be easier than reaching LH.