I always start by assuming that I will not get the upgrade! On that basis, I would rate LH (even with the old style C class seats) above TG.

Another factor against TG is that you need 2 of the UK vouchers each to upgrade on this route - LH will just require 1 voucher.

You might find it easier to get into F if you fly LH from KUL or BKK rather than SIN (KUL in particular is pretty easy and cheap to do as a separate ticket).

In any event, enjoy the holiday and let us know how you get on!