Interesting. Due to ADHD, I've been prescribed modafinil, and I've been prescribed various amphetamines (including adderall). My own experience is that the modafinil seemed to stop working after a while; it definitely did not seem as helpful as the amphetamines. Of course I'm just one case, and there are any number of reasons my own self-observations could be unreliable, but the drug industry does have a history of frequently making unfounded claims about the superiority of more profitable new drugs over less profitable old drugs, so I'm always fairly skeptical of newish drugs anyway.

[additional note: I just checked gwern's writeup, and discovered that tolerance is apparently a known issue with modafinil; it's not just me.]