They both have great mods, but New Vegas is definitely more active in the modding community than Fallout 3 is. Most Fallout 3 mods are no longer supported as well. Also New Vegas mods are much more stable than Fallout 3's. I guess I would have to give it to New Vegas in terms of quality overall, mostly since a lot of people started modding on Fallout 3 and learned a lot from it and made even better mods for New Vegas.
NV has better mods, you can even play fo3 inside the NV engine since they were the same engine just slightly modified. http://www.taleoftwowastelands.com/ its in alpha stage runs decent so far for me seems more stable playing fo3 inside NV than what fo3 was stable wise
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