BRU-ORD in C with upgrade to F: what to choose

I need to fly BRU-ORD. I can fly on 3rd Nov or 4th Nov but I have to be in ORD by 4PM on 4th. I can book business flex (C or D)

I can fly on the 3rd via MUC on LH434. Upgrade to F is already available.
I can fly on the 4th via ZRH on LX8. Upgrade to F is not available at this stage. LH434 not an option that day as I would arrive too late in ORD

Flying on 4th would be better but what are the chances an upgrade becoming available on LX8? Would you prefer to fly LH F or LX F?

Thanks for your advice