That depends on your experience with ict. If you are a dev, then there is no problem with Osmand, Modrana etc. But for non-devs, not very tech savvy people, OSK is more complicated to install than MagicEarth, that works just out of the box. You only need to install maps of your choice and a voice within the app. Of course fully open source is best, but userfriendlyness is very important too.
I first bought a Jolla 1 and used Here. Herwego isn’t open source either. If the Jolla 1 hadn’t been possible I probably never had got the courage to flash Sailfish on a new device.