Others may beg to differ, but UA's F seat is not much different from its J seat except wider and in single style. On the way to CDG I would want to maximise seep rather than trying out all perks of F (which on UA are pretty slim anyway) otherwise you'll be shattered when you get to Paris. I've personally not taken the TGV from CDG to Strasbourg so I can't tell you if that's a non-stop service or not, but TGV is pretty good in most respects. I would go for first class seating there since these trains are often heaving, especially during holiday times, which in France is quite frequently.