Microsoft will obviously spend tons of $$$ on ads for Durango. I don't know if they will spend money to hype up an event or spend money once it launches. I say both.
Either,
E3: Reveal
Q4: Launch
Or
March/April: Reveal
E3: Launch
Why else would they have a timer if it isn't going to be revealed at E3? If its announced before E3 then either plans have changed because of the PS4 announcement or it will be released this June.