it depends - use your intuition?
Great specific question, Borisoff. As you say, you've 1) used context (you're at your computer, so look at that list) to do the first round of decision-making. (Naturally you might also be near a phone, and have @Calls you could make, but let's stick with your example.) And 2), you know times available and needed, so now energy and priority come into play. I think the gist of GTD "doing" is that you need to integrate all your choices in your head, and make a call. If you're working during your "prime time" (am for most people), then you may want to tackle one of the "harder" ones. (Note: I can't tell whether "longer = harder".) I sometimes use my internal anxiety meter to determine which to work on - it's based on deadlines coming up in the calendar, which is hardest, which I'm procrastinating on, etc.
Hope this helps a little!