Well, it's really a bit of a toss-up...
Of course, Diablo's system has a bit more realism to it, but it's also much more cumbersome. The arrangement of items in your pack in the middle of a real-time battle adds to the confusion, and instills the concept of space conservation. Indeed, Diablo forces you to be much more conservative with your resources... but that's just it's brand of realism. With the rather simplistic playout of any hack-and-slash titles, the designers have to resort to either spectacular graphics, or on gimmicks and attention to realistic detail like this, to give them appeal.
Fallout is the exact opposite. Where Diablo has, comparitively between the two, a more realistic items system, Fallout focusses more on true-life gameplay. With the way Fallout games are designed to play out, having to worry about such micromanagement would be more burdensome than it would be gameplay-enhancing.
Ironic, considering that the Fallout world is supposed to be one of sparse resource, but in the end I both systems work best for their respective games. Honestly, who in Fallout would like to be limited to what they could carry? That's, what, a couple of holstered guns, two slung rifles, and a pack with ammo and a few other amenities? Conversely, would Diablo be as fun if you could freeze the game and take as much time as you needed for inventory? And, for that matter, if you could carry almost everything that ever dropped, would it really be the same game?
I guess the simple version of what I'm saying is, my preference really depends on the type of game I'm playing.
"Nil Desperandum"
http://fallout.gamestats.com/forum/User_files/3a5b0768718cafc4.jpg