New Vegas was excellent, a much better story and I would agree it was a slightly better game overall. I wouldn't say it was all that different from F3 though.
The DLC's were a mixed bag. I loved some of them to bits, but I couldn't stand the UFO one.
Not all that different? I guess it depends on what you're looking at. The engine was essentially the same, but that's not really all that important imo. As you said, NV's story was a lot better, which is really the heart of an RPG. It also offered ammo crafting, hardcore mode, some tweaks to various features, especially VATS, better UI for companions, and was developed by a studio that knew how to write dialog (Bethesda really does not).
Interestingly, Mothership was actually my favorite DLC for FO3
Everybody good, plenty of slaves for my robot colony?