I like Fallout 3 as much as I do New Vegas for very different reasons. After putting 250+ hours into Vegas and then taking a break for some weeks, its nice to reflect back on both games and recall experiences from both. For me I found the open-ness of Fallout 3 very enticing and rewarding, overall its a more polished game than New Vegas and that shows when you go back.
I agree that mods can make for an extremely enjoyable Fallout 3 experience now - most of the big mods are "finished" and in a stable, production release. Its not such a hard thing to put MMM, FWE, EVE, Fellout/Improved Weather, WMK, etc, etc without getting many conflicts or game problems - and a mixture like that really does change the Fallout 3 experience bigtime.
For modders I am having alot more fun with Fallout 3 than I was with New Vegas, but that can largely be attributed to the fact that it's still somewhat early in the FNV modding cycle. Some stuff doesn't work still, and folks are figuring things out. When I went back to Fallout 3 modding I was immediately pleased by the fact that everything works well there - the GECK and support tools are smooth and not missing functions, the environment is easier to load, test and manage than Vegas and all my object palettes still work!
In either case, no matter what aspects of the game you enjoy, I think there is still mucho fun in Fallout 3
Miax