I have 28,000 miles on Skymiles, but will be flying almost exclusively AirFrance (8-10 transatlantic flights/year) for the next year or two. Is there a big difference in the amount of points AirFrance gives to Skymiles vs FlyingBlue for the same segments? Should I switch?
Flying Blue is not a generous program particularly if you have the misfortune to be travelling in economy class. In the UK it is sometimes nicknamed "Flying Poo".
The main advantage is that you will have Flying Blue Gold after only thirty individual flights per year. (I assume you mean "8-10 return trips per year", if you are only doing 8 flights in total it is probably better to forget about frequent flier schemes and just take the cheapest flight.) Once you factor in connections and the odd unexpected trip the number of sectors will soon surpass that. Flying Blue Gold is the mid tier that gives you access to a range of mediocre-to-poor lounges, and SkyPriority, allowing you to skip the queues (or rather, join different queues). These benefits apply on Delta as well as Air France and the rest of SkyTeam. To get this through Delta would usually take more individual flights and now a minimum spend threshold.
The disadvantages are that redemptions are expensive in both miles and money (fees will run into the hundreds of euros), it is very hard to generate miles through alternative routes such as credit card spending, there's not much in the way of dedicated customer support, and you'll be at the back of the upgrade queue on Delta domestic flights.
To be honest it's much of a muchness really but I doubt there is anything compelling you to switch away from SkyMiles. Air France remains a tier 1 partner of SkyMiles and most of their flights will earn miles with Delta.