Borderlands uses the UDK, so it's not like they coded the engine from scratch

- but anyhoo, we can't surmise what the development cycle was, they are both very different games anyway.
I love them both, but Borderlands has co-op, but Fallout3 has a great storyline, but Borderlands has over twelfty-million weapons, but Fallout3 has great weapons, but Borderlands...
Keep on like that and you'd have an aneurism!.
The bottom line is that if you like Fallout3 then you'll probably like Borderlands, and vice versa - but I would say that Borderlands has a much wider appeal, non-RPG folk are more likely to enjoy Borderlands than Fallout3 I'd say, it's more accesible.
The way they play, well it's opposite ends of the spectrum - Borderlands has quite a lax attitude towards collision detection and accuracy, it's more like an MMO and an old FPS mixed together. Fallout3 is more intense, you have to take your time and be stealthy, usually not a good idea to go in, guns blazing. I couldn't choose between them, they are both vital and unique games IMO.