For me, it comes down to the vibe and atmosphere. Fallout 3 feels post-apocalyptic. Style-wise, they nailed it.
New Vegas is bright and colorful. Never gave me a feeling of fear or dread.
Sometimes more is just more. I never once felt while playing Fallout 3 that it didn’t have enough of something. I had plenty of fun just exploring the wasteland and didn’t need more story lines to push me in any direction.
Maybe I’ll give New Vegas another try since it’s cheap, but I still haven’t played Fallout 4. Not sure I have the attention span to do both.