It mostly comes down to stability. Manjaro is based on Arch, and while they do test the repositories before sending it out to the public, It's just a very large repository to fully test. They also seem to be dedicated to stay as close to the bleeding edge of the Arch Repositories as possible. They want their users to have as up to date software as they can give them whilst maintaining stability. This is great, but it's a delicate balance to be walking and occasionally a mistake is going to be made given the size of their repositories. Updates on Manjaro aren't necessarily likely to give you problems, but they're much more likely to give you problems then they would on Solus, and it would likely depend on the software you have installed.
Meanwhile, the Solus team has put a significantly greater priority on stability and curation. Due to the nature of how they manage their systems, and how small their repository is (by comparison to Arch), it is much easier to be certain of a stable build before they push it to us users. There's just less things that need to be checked. The Solus library may be small, but there's virtually everything you need in it if you are most users. The Solus dev team has done a phenomenal job of curating their repository.
So it really comes down to this. Do you want a slightly less stable distro with a larger slightly more up to date repository, or a decently more stable distro with a smaller, somewhat less up to date repository. Stability, or the bleeding edge. Curation, or a gargantuan collection. I personally choose stability and curation, but your mileage may vary. I advise giving both a quick whirl before you decide (if possible), or at least try and find some footage of people using them (preferably on real hardware rather than a vm).
The last thing I would say, and this may just be me being a little paranoid, but if you do decide on Manjaro I advise always checking over on their forums or wherever they track bugs everytime before you update. Just in case.