It depends totally on what you want. If you already have the Persian Gulf map, NTTR for a desert map means a bit less than it would otherwise. That bit is up to you, so I’ll just compare the aircraft.
Both birds are delta wing (MiG-21 is a modified delta), so they both like to get fast and stay fast. Both can accelerate like nobody’s business, and the delta wing lets them keep that airspeed without as much induced drag. Both can make a high energy turn on a dime, but you will bleed to stall speed within 180 degrees if you attempt a bat-turn. Both planes are Fox1 capable (though the Mirage radar is better and easier, and the Super 530 is better than the R-3R and way better than the RS-2). Both startups are easy, though the Mirage takes longer because you need to align the INS. Otherwise, the planes are completely different.
The MiG-21 is a stick and rudder rocket sled that will scare the crap out of you at least once or twice every flight (not counting landing, which is a guaranteed scare). There are rudimentary autopilot controls (a dampener, low level pitch-up, and altitude hold), but it’s a high maintenance aircraft in that you have to keep track of a lot of things in a less-than-ergonomic cockpit. And that’s just flying the bird; fighting it is a totally new level of task saturation, not to mention setting up your pipper and switches properly for every weapon (selecting the sight and ballistic settings, selecting the seeker type, and selecting not only the proper station but the proper station corresponding to the proper mode on the same dial). Switching from missiles to guns in a turning fight can be so difficult it’s often easier just to keep eyes on the bandit and use Kentucky Windage to aim the cannon. The most fun I’ve had with the MiG-21 is flying that temperamental beast at low level, using the RATO for take-off, and firing the Grom at targets. All of those alone are worth the price of admission.
The Mirage is one of my favorite modules. Aside from being a delta wing, it is the opposite of the MiG-21. It is a fly-by-wire aircraft (well modeled by RAZBAM) with a G-limiter and it is easy to fly at high speed and low altitude. The autopilot is intuitive and powerful, and while it’s early 80s technology, you can do just about anything you want with the trim hat. Quite simply, if the Mirage scares you you’ve likely done something wrong! I usually recommend the Mirage as a good gateway for full-DCS modules, as it is a good introduction to a full-clickable cockpit while the autopilot allows you to explore the cockpit without worry and the systems are simple and intuitive enough to mostly set and forget and focus on fighting. Finally, the approach mode in the Mirage is nice, especially if you have the airfield as your final waypoint (it will give you a runway outline in the HUD for poor weather approaches). The HUD cues give you indications of acceleration (the two carots relative to your FPM, just like the regular HUD modes), and a staple gives you an ideal AOA to give you a feel for landing a delta wing. Just line up the FPM, staple, and carots on the runway threshold and ride it in, and you can also develop a good sight picture for the MiG-21 as well!
Ultimately, it boils down to whether you want a 4th gen FBW fighter, or a stick and rudder challenge.
