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Short answer:
Don't bother with FO3 unless you're prepared to invest time installing mods to improve the experience. The questlines, dialogue, and gameplay of FO3 seem primitive by comparison to Obsidian's work.
Longer answer:
I view FONV as an evolution of the gameplay and concepts introduced in FO3. I've played both games extensively, and wouldn't feel comfortable moving backwards. In its vanilla state, without mods, FO3's gameplay feels somewhat primitive by comparison to FONV. Much of the gameplay I enjoy in New Vegas, only becomes available in FO3 via mods. Specifically: Hardcore mode, iron sights, reduced meds; limited ammunition. To my knowledge, no one has improved the shallow writing and character development via mods.
If you feel comfortable investing a lot of time tweaking and modifying FO3, then you'll probably enjoy the gameplay experience. Otherwise, stick to FONV.
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