Login v/s Logon

We choose 'log in' for our more technical (vs. consumer) audience, but I agree that going with the standard that your particular users might see (80% makes 'standard') is the way to go.
On the one word vs two word question: English language usage dictates that when used as a verb (action), it's 'log in' or 'logged in'. When used as a noun or adjective (what is your login, or login information) it's one word.