I used GaiaGPS for my Android for a while, but I kept having some problems. To be clear, these might be limited to my particular phone --- the variants of hardware plus operating system software in the Android world is huge. But for whatever the reason, sometimes I would cache map data and be out 'in the field' and part of it just wouldn't show up, no map data arbitrarily for part of where I was at. Or there would be data, but not at the zoom level that I wanted.
I also had trouble saving the map data on my actual (not the weird sort of pseudo) SD card, which on my lower end smart phone is where most of my useable storage is.
So I switched to Backcountry Navigator, and these problems went away. It also seems to have a slightly richer set of free map downloads, to include aerial images. I did have trouble downloading one type of aerial images (don't use these much, but for fun and/or in-the-field recon ...). Customer service was great to work with --- in fact it seemed great with both companies, and ultimately neither really solved my problem. The GaiaGPS problems were, however, more fundamental, and the Backcountry Navigator download problem was "solved" for me when they told me that another type of aerial would likely download fine for me --- and it did.
I think it's tough to be really objective about this stuff as we each mostly just know our own particular hardware and software combo. I think they're both good products, just for me at least, BCN is the one to use.