Have been following this thread for a while now, and it has been helpful, so I thought I would share my own experience. I had two falls in the last two years with head impact.
Last year I had a slow speed silly washout on the easiest trail around. I was wearing a Giro Chronicle MIPS. The way I fell (by complete surprise, with my hands still in the bars), my temple got the main impact resulting in a concussion. It did not feel like a strong impact, but it really ringed the bell. Symptoms lasted for five weeks, and that scare set me in the path of searching for better protection. The Giro helmet was a very comfortable fit, but for a low speed impact I felt it did not absorb that much energy. So after that first hit, having a helmet with some sort of low energy impact absorption became a priority. I also wanted to have MIPS as the concept of avoiding rotational/shear forces makes sense to me (on my first hit the force was mostly perpendicular to the helmet on the side, so I think MIPS would not have helped on that case). After much research, last year I ended up buying a Smith Forefront 2 MIPS, mainly due to the Koroyd technology. I recently read this crash report (
Crash Tested: Smith Forefront 2 Helmet Review | The Loam Wolf ). Overall think that is one of the best options up there for a half lid.
Around the same time I also got a Bell Super 3R MIPS Medium. I wasn't looking for a convertible helmet, but found one for $49 at Sierra and since it was a good fit I decided to give it a try. I have been happy with the Bell, and have been using both the Smith and the Super 3r over the last year. Even it didn't have a system like Koroyd or dual density eps, the Super 3r was so comfortable and felt more solid than the Chronicle with the extra face protection, that I would use the Bell on some of the more difficult trails. A couple of months ago I finally got to test it, I went OTB hitting my head and left shoulder. This time the impact was much bigger than the last time, but I think the helmet (and MIPS) did their work. Had very mild nausea for 3-4 days, but overall got a much better outcome than the last time (and also avoided me a visit to the dentist).
So after that second fall I went back again to the search for a new helmet. This time I knew I wanted a full face. I did try the Bell Super Air R (as I was hoping for a lightweight helmet with a similar fit as the 3R with MIPS spherical, but it had a weird loose fit around the jaws) and the TLD Stages (not that great fit for me), but I ended up getting a Smith Mainline (20% discount at REI), and also a cash replacement Super 3R MIPS. So right now I have three options that I use depending what I am riding that day and the conditions. You can see below a side by side comparison as well as weights for the Smith Mainline, Bell Super 3R and Smith Forefront 2. All of them Medium (my head circumference is 57cm).
Overall the Mainline has become my new favorite, at almost the same weight as the Super 3r (with light support), it feels so much more solid in the head that I have been using it even on easier trails. It is so comfortable that is hard to find a reason not to use it. Sometimes I do feel silly wearing it on easier stuff, but with a history of concussion, questionable skills, and in the middle of a pandemic, it is a no brainer. As it was mentioned before, the best option is not to fall, but as long the risk is there, it is going to be the Mainline most of the time for me, the Forefront 2 for hot days or longer less technical days, and the Super 3r on those few cases where a convertible would make sense.
PS. I also considered the IXS Trigger Full Face, but the lack of MIPS was a no-go for me. Just got an email last week that the MIPS option is now available.