I do own the SYM 70.
I did own the Kapex 120 for quite some time.
I briefly owned the Kapex KS 60 - less than a week.
I have a Metabo 18V KGS 216 S (sliding) mitre saw - which is pretty good I would say.
I won't go into lengthy comparisons between the three Festool saws and/or other machines. In short I liked the Kapex 120 very much but it was a bit too large to haul around for my on site use. I do miss it at times.
I didn't like the KS60 that much and it does nothing extra for me and it does not bring anything useful to my workflow that I can't get cheaper, as good as - and in a lighter and smaller package. I actually much prefer the Metabo 18V to the Festool KS60 though build quality on the Festool is better, for sure. Then again I don't think the KS60 is built as well as the old Makita LS0714 saw which is an old favourite of mine in that blade size segment.
If I was going to get one mitre saw and one only I would probably go for the Metabo SYM myself. It is a well thought out machine and I have seen it in the flesh.
The 216mm blade size is the sweet spot for me and that's what the SYM70 and the Metabo shares. If I ever add a mitre saw to the stable it would probable be another 216mm blade machine, most likely that Metabo.
The SYM 70 I could replace with a few other options but it just so happens that I like it, light, simple and accurate. I run it on a plug it cord and when it comes down to a few pieces of smaller trim at odd angles it is great to have it on site or in the shop.
The price of the SYM70 today is simply put; way too high. I bought my second one, as new, for less than half the retail price.
I have an aluminium blade on it and I do alu trim with it - superb.
Festool did not design the SYM 70 though, it is an old patent (Holz-Her?) and the saw has been around long before Festool took over the patent/design. The design is ingenious but dated. It would lend itself well to an 18V platform and with a laser and LED light in a package at the same price and weight as the SYM 70.
In the subject of mitre saws I did also get a Makita 18V DLS600 which is a small 165mm blade, dual bevel mini mitre that has worked wonders too. It is also "limited" yet very versatile. And super light! I was going to part with the Festool SYM 70 when I got the Makita but since I occasionally do alu trim I decided to get a good Bosch Alu blade for the SYM 70 and keep it. Made sense to share blades with the Metabo and that Alu blade was changed over to the Metabo yesterday for shortening a track saw rail that got dented at one end.
Point is that I have - again - piled up on a few mitre saws for versatility though I could probably replace at least two of them with that Metabo saw, but I could not replace any of them with the KS60. The Metabo is for me what the KS60 should have been.
Also that Metabo can be paired up with the 18V saw and share blades which gives the Metabo and additional advantage over the KS60. Sure, you can share the blade from the KS60 with the Metabo as well but you are not getting that SYM feature.