Black Shark 2 *AND* A-10C
Black Shark offers a much more challenging flight enirvonment - the learning curve on basic flight is a lot higher than your typical flight sim. The weapons systems are, by comparison to the A-10, are simple, but still requires systems-level knowledge and practice. Maintaining flight and employing the weapons systems simultaneously in a combat situation can be a challenge.
A-10C offers signficant depth in systems knowledge - DSMS weapons profiles, flight plan management, SOI vs SPI, CCIP vs. CR vs CCRP, datalink messaging, etc. - but flying the Hog is much simplier than flying the Ka-50.
I think learning to fly should come before learning to fight, but learning to fly the Ka-50 won't help you fly the DCS A-10C. I guess it comes down to which system do you like more?