FC3 have a much more accessible frame work to learn. Turning on the aircraft and fly into battle zone is much faster than full simulation modules or whatever people want to call them.
FC3 aicrafts in majorty already count with PFM which enhance their flight dynamics over previous version of FC. They are great btw but still beta for some.
-Speaking on fighters: there are no generation 4 fighters on full fidelity modules yet, but you have in FC3, which are extensively used in multiplayer. The closest you get from FC3 fast movers is DCS MiG 21 at the moment, but it is a 3rd gen fighter and, mathematically, it should get it's butt kicked by any FC3 fast movers. But if you're an adventurer that wouldn't be a issue (like me and many others). XD
MiG-21 (I don't own) for what I know: it's a very well received module. Check some youtube videos of players taking down Su-27s and F-15s with it in multiplayer. Kill/Death ratio is poor tho...but looks fun XD.
-Speaking on Ground Attack or Ground Support aircrafts: FC3 has the Su-25A, wich is a cleaner version of Su-25T that everyone gets for free. I personaly got the 25A and loved it over the T for it old schoolness and the slight more feel of danger and low tech. It really is a beast to tame and a LOT of fun if you don't mind not having fancy electronics. Same goes to A-10A but you can count with a HUD on the same quality level as F-15C's HUD.
Under the same topic: "Full fidelity" DCS:A-10C. This is the master of DCS modules for me in the moment, even though I don't fly that much anymore after heavy hours for months on it. I got it at beta and started playing after a year when I got a decent GPU. It took me a month to master my very first DCS module from ground zero. I haven't yet found any other sim with such quality and complexity as the Warthog taking from the nature of the real aircraft and details put into the module.
-Helos Modules (I own them all): There's the KA-50, that's not really my cup of tea, but I managed to get a lot of relaxing slow paced experience from it. It is REALLY good flying and steathing into darkish enemy territory, even more with a friend.
Huey and Mi-8: Both might look very similar but fly and "play" alot different. Mi-8 heavier and much more delicate to maneuver and the Huey, with very strong structure and higher agility, count with two miniguns that can be pointed by co-pilot view (you can change seats on these two modules) whatever you want. Both are amazing.
Sorry to bother about some full fidelity modules, but the point is, they're very different in how to use it on the battlefield. One of them MIGHT be the thing for someone for a very VERY long period while FC3 wont hold much. It's a matter of personal taste and objective within the DCS World.
If you want fast deployment and high variety, Flaming Cliffs 3 is definitely the way to go. If you want DEPTH with still a LOT of fun, go for the module you find better for you. All play different. It depends on you in which one to go. The main site have a very long presentation for each different module by text and pictures for each capability of them.
There's no long enough comment here that'll cover everything you should know. :) Great flying and good luck.
I hope I've helped.