Interplay, all the way. This thread is biased, because many people would only know Fallout from Bethesda, and basically what they did was change the game so that it became a post-apocalyptic version of Oblivion.
The franchise was quite vibrant beforehand, but catered to a different playing style, which clearly has been lost since Bethesda took over the IP. There's more to Fallout than the setting, but since many of the people simply don't know, don't care, and are basically used to a different style of play this is all forgotten for the new-fangled versions.
The same is going to happen with X-Com - you change the gameplay, and you change the game. X-Com was originally a strategy game, but now it's being converted into an action shooter. This is NOT what people recall and remember from the X-Com franchise however - it's an entirely new game in it's own right.
Bethesda rebooted the Fallout setting, but created a new game that is doing a major injustice to the past by being called Fallout and considered the same game. That's like trying to compare Command & Conquer and Tetris, for crying out loud. But then, when it comes to making money and ripping off ideas, since when did the past ever matter in an industry which has obsolescence built into it with every new console generation and OS release.
To say Fallout hasn't aged well is to miss the point completely. RPGs evolved, with new types of gameplay, thanks to new things being thrust in with the RPG category, like Diablo and Morrowind, even though these are completely different games.
Diablo is an action RPG, Morrowind/Oblivion is a First Person RPG, and Fallout is an isometric turn-based RPG. Three different games, three different types of gameplay. Most people who make the choice between Bethesda and Interplay do so because they prefer one type of gameplay over another. That's your decision - that's your choice. But with such differences, it is hardly a case of the games not aging well, since all the isometric RPGs from Interplay are still some of the best selling titles on Good Old Games. It's just that, for the most part, RPG gamers have changed their tastes, simply because companies no longer see merit in isometric or turn-based RPGs, and have effectively abandoned entire swathes of the RPG community to try and compete with the latest fad.
This was the case with Lionheart, which saw a fantasy take on the fallout system converted into a Diablo clone, which turned many people off the game. You'd get the same response if Bethesda suddenly decided Elder Scrolls: Skyrim should actually be a side-scrolling platformer than a First Person RPG. It may or may not be any good, but it would be a different game, and that's what people would be judging it on.
