LH and LX transatlantic are both very good services, in any case much better products than WT+, which is a compromise service which delivers exactly what it says.
To take part of Phileas Foggs point, if you were ex-London and it was WT+ direct, or business via x or y, that might swing it in favour of WT+. But your are not.
AF in flight service is good, but the ground is a total disaster area, especially in Paris CDG where they will lose at least one of your bags.
For 400 pounds extra, go ahead and treat yourself a bit on LH.
