It depends a bit what time of year you fly, as aircraft used and timetable are different, and what matters to you. Let's assume during summer timetable perios:
- SEA-CDG has choice between DL 767 - which is OK, but frankly not more - and AF 777-200. Air France is *clearly" the better product between the two
- SEA-FRA is on the 747-400, SEA-MUC is on the A350-900. If you're traveling alone, the AF 777-200 is clearly better because Lufthansa still has a 2-2-2 configuration, albeit for a seat that is otherwise comfortable and spacious. If you're not alone, flying the 747 on the upper deck or in the nose isn't bad, and the A350 is a pleasant experience
- Service, food, etc. on the trans-Atlantic. Seriously, I know both airlines, it's a toss-up. This forum will tell you AF is better, in the Lufthansa forum they'll tell you that LH is better. The only real difference is that there is less suprise effect, good or bad, with LH
- Europe-CAI: well, that is where AF is clearly better. The A340-300 is a proper longhaul plane, albeit with a relatively dated Business class cabin (even if it's JOONed). Lufthansa uses A321s, Euro seats, and just keep the middle seat free. Having another couple of hours of rest in a proper Business class seat rather than not resting because of the uncomfotable Economy-turned-Business seat may be a tie-breaker
- Transfer airports: Munich is *by far* the most pleasant and civilised transfer experience. Some here will tell you that Frankfurt is terrible, but there are just as many people that find Paris CDG terrible. Both are mediocre
- So it may be down to schedule: and this is where Frankfurt is actually the best. In Paris, you would have a very long layover. Unless you like wathcing the paint dry on the lounge walls, 5 hours or more may be quite long vs. 2 hours in Frankfurt. On the return, similar story: 2 hours in Frankfurt, but 7 hours in CDG if you want to wait for the Air France-operated connection. IF you are fine with the DL flight, it's only 3,5 hours wait
All in all, if it weren't for the long layover in Paris, I'd go Air France 777 > Air France 340.