Without any particular reason, but I would book LH and then try to bid for an upgrade into F. Earlier this year (after toying around with the upgrade bid process, but not actually submitting a bid) I was offered by email a chance to buy-up from J to F at a very reasonable cost (my J cost plus the upgrade charge was less than the cheapest F cost I had seen in the 3 months prior to booking J). No guarantee you will be offered the chance to upgrade.... my flight was ORD-BCN via FRA