Early versions of the S&T Tar-21 had a very bad reputation and had some known issues. But I haven't heard anything about their newer version which seems to be of a much higher quality. The body texture of the new S&T Tar-21 is similar to the ARES version where-as the earlier version of the S&T Tar was different. Personally I would look for a used ARES since they can be had for around $200 or less. I've only owned the ARES Tar-21 so I can't say anything more than there being a few key differences between the two. I do like that the S&T version is wired to the front and has battery space in the foregrip where-as the ARES was wired to the rear and could only accommodate a small type battery or a buffer tube lipo.
G&P MK23 - SMP G&P MK18 - STAR FNC x2 - Tokyo Marui MP5 - CA M24 - UTG M3 - Cybergun M&P 9
I had an older version of the S&T Tavor for a bit. And it gathered a reputation for breaking tappet plates. Went through about 4. And since I couldn't find a Tavor specific plate, I modified what I had to work with it. But the version I had did have a metal gearbox/quick change spring guide. One of my gripes with it though, is it really could only take small type batteries. No larger than say a brick 9.6v NiMh.