I couldn't really get into StreetComplete when I tried it, but I think that's mainly because I'm used to the iD editor's UI and because it isn't fully featured. Vespucci solved both of those things for me and gave me a fantastic editing experience. That said, for all I know, recommending Vespucci could leave a newcomer completely overwhelmed with options. So I would say that it's worth starting with StreetComplete if you want a highly gamified experience for stuff like tag editing for existing objects or starting with Vespucci if you feel like you want something extremely powerful, then trying the other one if your first choice's UI doesn't suit you or doesn't do what you want it to do. (StreetComplete and Vespucci are both available on F-Droid.)

OSM has a ways to go to be entirely competitive with GMaps as a navigation tool in most regions (although it gets the upper hand in other areas). OSM's major advantages are four-fold:

• It's open to be used by anyone for any reason for free.
• It can be contributed to by anyone.
• (Crucially) It has a way higher ceiling than GMaps could ever hope to have. The level of potential granularity in OSM is absolutely insane. You can mark fire hydrants down to the color, diameter, pressure, and number of couplings. You can mark power lines down to the voltage, shape and material of each individual pole, etc. Individual trees can be marked down to the species. Every street crossing can be marked as having tactile pavings, a type of curb, a material, signals, refuge island, elevated or not, etc. Individual entrances to buildings can be marked as different types and with different door mechanisms. Heights of buildings in meters, whether they have air conditioning, etc., can be marked. This is barely scratching the surface. For navigation, things like this can be superfluous (I would argue that for people with disabilities like blindness, some of these things like the crossing types could be useful), but for research and specific applications, it can in theory crush GMaps rather than just being brought into parity with it.
• The non-satellite map is just way, way better. If I look at my neighborhood which is reasonably well-mapped on OSM and then compare it to GMaps and Bing Maps, the latter two look like an absolute joke and rely heavily on satellite imagery to fill in the gaps. The problem with that of course is that not everything is visible from space, and it often gets fuzzy with minute details.