Heh, I like these ones.

Well, traditional *nix prompts when you conect are always "Login:", so if you are looking for a precedent that's probably it.

I (like many IT bods who've been in the industry for a while) tend to mix the two terms depending on the last one I read.

I suggest that you pick one you like and stick with it. If anyone complains, roll your eyes and mutter "pedant" repeatedly until they stop

However, in the spirit of livening up the debate, here are two different pseudo-etymological reasons for the differences, neither of which should be taken seriously:
  1. Login indicates a process or action. It is the action of authenticating using credentials, usually an id and password pair. The antonym is "logout".
    Logon is a process state. You may "have" a logon.


    OR


  2. Login indicates the process of local authentication to an individual machine, i.e. you "login" to a *nix server, router or stand-alone PC to get into it.
    Logon indicates the process of remote authentication to an amorphous resource, i.e. you "logon" to a domain. You are not directly attached to it

    You are in a car, but on the road. Yes, I know it's a sophisticated and devisive analogy, but it amused me


Interestingly, the Webopedia entry only has the term "log on" as seperate words, with the synonyms being "log in" and "login" - there is no "logon".