I did a similar sort of trip last year with Delta/Air France in Economy;
MAN-ATL-MIA Delta 777/764
MIA-CDG-MAN Air France 742/British Airways 735
I flew MAN-ATL on a Delta 777 and the legroom was poor, but this was compensated for by the fact that the midweek flight was only about half full in economy and I had 2 seats to myself, which meant I could sit diagonally accross the seats and could keep the airshow permanently on one TV screen. Delta's 777s have footrests in economy, but you'd have to be pretty short to use them!
The 767-400 on the ATL-MIA leg had noticably better legroom, and the phone/entertainment controls were also much better placed, I didn't keep accidentaly changing channels with my elbow like I did on the 777!!!
Returning I flew Skyteam partner Air France on a 747-200 MIA-CDG and the legroom was pretty good, but not as good as the British Airways 737-500 for the CDG-MAN leg, which had all leather seats and legroom comparable to Air France - and I was right at the back in the proper economy seats rather than unused Club Europe ones...
You'll find service on Delta a bit hit-and-miss. Food is OK, and on Trans-Atlantic legs they're quite generous with the alcohol. Main point was the cabin crew, who ranged from very friendly to borderline rude, but on the whole were cold and efficient rather than warm and friendly. Crew on the short ATL-MIA leg were actually better, but of course food service was a sandwich in a paper bag and I've had better!
Flying home on Air France I'd heard the rumours about their surly service and poor treatment of non-French travellers, I was prepared for a bad flight. However I was pleasantly surprised. The Air France crew were quite charming and pleasant, the service was very French and I suppose some people not used to French people may have regarded it as slightly brash. Air France's crew made very frequent passes through the cabin during the night with bottles of water and the cabin crew were always on hand, in stark contrast to Delta's crew who were totally absent from the cabin during the day flight for up to 2 hours, and never offered water once. On a safety note the Delta crew never made a seatbelt check before landing on the MAN-ATL flight and people were getting up to use the toilets while we were on final approach to Atlanta.
Hats off to BA though, excellent crew, well there were 4 of them and only 40 of us on the 737! But we did get a hot breakfast, tea and coffee twice and inflight sales on a 55 min CDG-MAN flight. Delta only managed one drink on the 1h 30 min ATL-MIA flight and if you wanted anything as "exotic" as beer you had to pay for it!
While Delta didn't provide a bad experience, and I wouldn't mind flying them again, I wouldn't go out of my way to. Air France on the other hand exceeded my expectations, even thought my flight was operated by a B747-200 instead of the scheduled B747-400. I'm sure the B777/A330/340 with their PTVs are even better.
Don't let the negative comments about Air France's service put you off...
As for Business Class, can't comment, can't afford it!!!