Unless the UA Suite is absolutely essential, I would go for LH for their far superior FRA terminal experience, better service and much more elaborate food offerings. UA might be fine for Y or even C, but I felt that they really lacked the extra "something special" in F.

Seating can be awkward if you're on a 747 with no companion on F because of the double seats config (A330/340 has single seats down the side), but for me personally I don't feel it's a huge problem.