I would recommend the AF product over UA if you are travelling from SFO to CDG. Assuming you are talking about Seat Plus, rather than Premium Economy on AF, then the size of seat and level of service are broadly similar on AF as UA's E+, except they give you a drink on AF and the food is not quite as vile.

One advantage is the top deck of the 747. This is significantly quieter than the lower deck and a much more pleasant and restful experience than anything UA can offer.

But the big advantage, particularly eastbound, is that you don't have to change anywhere. Whilst a change in LHR, FRA or MUC is bearable, changing on the East Coast or midwest is horrible. You get two long flights, instead of just one, or one and a short hop, and any sleep you do get will be badly interrupted. IME, it adds days to my jet lag recovery.

Finally, if UA can't be bothered to fly nonstop to CDG out of one of its main hubs, it doesn't deserve the business.