Same engines, mods are great with both on PC, but this a hard question, to answer indeed so I have to default to my opinion and preference with Elder Scrolls series (personal preference) and yes the Elder Scrolls Blades is actually fantastic, looking forward to beating it senseless eventually ...
Fallout was always fun for me but I started with Elder Scrolls series myself.
Tried both and love both. They both fit my aesthetic so much! Love Fantasy, Magic, Dragons and more? Elder Scrolls! Love Sci-fi, Guns, Robot, and the 50's? Fallout! Either way, both are made under Bethesda, so I love em both just as much!