As Italy or other nations with little IC should I do INT or FIG? As Germany or Uk or Ussr is it worth producing both?
Figther or Interceptors?
I went with interceptors for Germany and fighters with China.
Fighters are more versatile: If they aren't needed for air superiority, they can assist the army. They have a very low soft and hard attack, though. But it certainly helps raising experience and levels of the commanding officers very fast.
Fighters have a longer range. That was the deciding factor why I used fighters as China, distances are rather long and often enough an interceptor couldn't even had reached adjascent regions.
Interceptors are cheaper and better in their core role: fighting enemy bombers. They are so good in it, that it would be a waste to let them do anything else as long as there are enemy bombers in the air. They are an about even match for fighters. Later on they also get a lowly soft attack, so in theory it would be possible to use them, similar to fighters, also in gound attack/support missions.
Tech-wise the newest interceptors can be researched 1 year before multi-role fighters.
For Italy:
Depends what you plan to do. If you only want to go for North Africa, distances there inbetween regions are rather small, so interceptors are fine.
If you want to conquere all of Africa, you'll need fighters and TAC because distances inbetween regions get vast and interceptores/CAS don't have the range. Especially vs the lone enemy militia in a desert or jungle which can be only reached by a 6-month tour, TAC are great. It might take a while to get that unit wiped out but much better than trying to encircle a lone militia in a 3-6 month campaign.