I've found New Vegas much stabler and just better put together than FO3. I've heard so many people complain that NV was buggier than FO3 but I've never had a single game breaking bug in any of the NV DLC or the base game (except corrupted saves, which applies to both equally). I have had to fix myself up in the console or by repeatedly reverting to CASM saves multiple times in Anchorage and Zeta when whole quest chains got skipped and left me stranded in script-limbo. I've looked each of these bugs up and every single one is a verified bug for the DLC on vanilla FO3. Not for the reason you might expect (New Vegas/Tale of Two Wastelands), though. The one thing that has been most surprising is how many game breaking bugs I have encountered in the FO3 DLC. The Pitt felt very functional in TTW but I think that DLC has some other non-balance-related problems. The escape at the start of Mothership Zeta is almost impossible, the enemies in Point Lookout are just totally nightmarish, and in both Zeta and Anchorage the biggest challenge is just staying hydrated.
I have never played any of them in FO3 but I have to imagine that they are much smoother without the NV overhauls on top. The FO3 DLC are really unbalanced in TTW with JSawyer and PN installed.
