For me, it isn't the software/firmware/hardware comparison, it is what I know and what I will have to learn. I don't want to invest a lot of time in learning a completely new system of developing, storing, uploading, modifying and using maps, only to find out it has the same or other limitations as the system I am currently using (Garmin). I am as frustrated as anyone that Garmin develops systems that don't even work together (Basecamp/Zumo), but my limited experience with other systems (Google/Android, a couple of windows apps) is they have their own limitations.
My Windows phone allowed me to download maps for offline use as large as the SD card could hold, but had only a few klugy apps that worked with it. My Android/Google phone asks me for a "box" that is only about ten miles square to download for offline use. It doesn't take long to use that up.
I also found that because of the volume of Garmin devices in the market other entities such as OSM develops interfaces to work with Garmin. That's a big plus.
Finally, I am not sure I trust my electronics to attempt a single point of failure. With a phone doing everything, should it fail, get lost or stolen, everything is gone. Mapping, Phone, Text, email, etc. I guess I could carry a second phone, but then why not carry a GPS. Until something comes along, and takes the consumer market by storm, I am pretty much a stuck in the mud, complaining about Garmin's overall system approach.