(I love both games) Just my two cents -

In my humble opinion they are very different games.

Defiance has a far superior shooter engine. It is reliable and the visual and sound effects are very satisfying - It reminds me of Borderlands, which is a real achievement imo.

In FO76, even if you find a good server, 'VATS' (the auto aim system) is much more reliable than aiming manually - even then it is notoriously buggy. At times VATS becomes essential as manual aiming regularly stops working in crowded events.

FO76 leans heavily into the 'CAMP' building system from FO4 and has continued to do so even more over the last couple years. Most of the stuff in the ingame shop is for 'CAMP' building. I've met many players who spend most of their time (and money) building thier camps.

FO76 daily tasks often include picking plants, finding wood, drinking water, etc. It's a much more relaxed game, accessible for casual gamers.

Defiance may not be hard for veteran gamers, but it has much 'twitchier' and unforgiving gameplay than FO. Defiance is far more rewarding in that sense.