I would favor the short hop on AF or BA personally. You’ll need to clear customs to access the arrivals lounge. Then transfer via tube/heathrow exp to t4 (AF) or t5 (BA). Keep in mind you’ll still need to arrive 60-90 min before departure to st pancras for a train due to customs and airport-like security. So I think flight would likely be quicker, however with the train you’ll be arriving tight into GDN in Paris as opposed to out at CDG.