Personally I like the Scar because (without bling) I can use a silencer and the iron sights (Left Trigger look through is pretty accurate). In my opinion the Tar-21 is stronger and had more bullets but has a lot of inaccuracies due to gun moving around.
Tar-21 is more medium/short range. If you have Steady Aim with Red Dot/Holographic/ACOG it can be accurate at long ranger.
Among the Assault Rifles I would say TAR-21 is a good chocie if you are looking to replace your SCAR (i.e. finished all the challenges)
the tar-21 has quite a bit of recoil per shot so overall accuracy is quite reduced while using rapid fire, not that its accuracy is any good even with a controlled burst, or even single shot fire. however the power makes up for that. its more of a short-mid range weapons rather than mid-long range. so i suggest you stick with something else for now.