Personally I prefer TES, it has a much more interesting story and lore, Fallout's never interested me that much. Gameplay can be a bit hit or miss, but VATS is definetely fun as hell in Fallout.

I think Fallout has better characters sometimes, they can be pretty funny. TES characters usually sound the same so it can be a bit hard to tell them apart.

In terms of things to to and replay value, TES has the edge. I can be a stealthy archer, a battlemage or a tanky sword swinger. Plus the world is so damn big I'm always finding new shit to do.

Gameplay - FO
Even though Fallout combat is sub-par already, TES is just shit tier.

Lore - TES
Actually took some world building.

Things to do - TES
Fallout is just too combat oriented.

Characters - FO
The interesting characters in TES are more or less just newspaper funnies in terms of depth.

Replayability - depends on how you value roleplaying or combat.

^
yeah what he said

Fallout in all categories. I just much prefer post apocalyptic stuff to fantasy. The universe is just so much more interesting and unique. Both are great though.

Bethesda does TES better than fallout.

So TES

Fallout. Has a much more engaging lore and universe for me.

TES because I enjoy fantasy more

-Gameplay

For as shitty as what the gunplay in Fallout is, I prefer that over the shitty depth-less combat system ES uses.

-Lore

I'm going with Fallout due to it being slightly more interesting.

-Things to do

Both games are tied for this since I never role played in either game. 

-Characters

Fallout due to Fallout 2 and NV. The majority off the characters in the ES are extremely depth less and all the companions may as well be the same.

-How much time you spend playing/Replay Value

Elder Scrolls. I have more replays in Oblivion and Skyrim than what I do for Fallout 1, 2, 3 and NV. Fallout 2 I still haven't beating along with NV and F3 I only beating once. For dicking around I spent more time wondering around in the ES than what I did for Fallout due to the environment looking far better. In Fallout everything is destroyed an it looks the same which I find boring to wondering around not doing anything.

I will say I find it easier to RP in TES than in Fallout. I guess there's something about a world where the PC gets abducted by aliens, peoples heads explode after taking a bullet to the face and a system that allows you to pause time to let RNG decide your battles for you that kills some immersion for me. I still massively prefer the lore - I generally find post-apocalyptic settings, particularly one as unique as Fallout's, more interesting than high fantasy.