Yeah, but it's all in the writing,
SM!

You make a good point there, but only slightly. Ripley had time to suit up in Alien because the alien sleeping didn't detect her, but would have been desperate to escape the Queen in Aliens, probably willing to sacrifice herself if she had to to save her comrades, and did what she thought was the only solution.
But she has time to suit up into a powerloader. When she comes out of the closet to face the Queen, she could easily have been shown wearing a vacuum suit as well as the powerloader (maybe as an integral part of the design of the powerloader, even)
if Cameron was worried that the end wouldn't play out. But it may well have been another one of those technicalities that only a microscopic percentage of fans/audience would take issue with, that got sacrificed for heightened drama during the Ripley/Queen fight, etc.
Cameron & Hurd on the airlock scene:
Lofficier: Why reuse the ending of the first film, I mean, expelling the alien into space?
Cameron: It seemed the only way to go. There was no other way that satisfied me. Crippling her to death would have been impossible. Remember, the acid blood... The image that I could not shake free was the idea of literally hanging, being suspended over an infinite abyss. It was not so much getting rid of the alien as the jeopardy that Ripley was in after the alien was already out. I wanted to have the image of standing along something, and the doors open, and there is nothing there but stars. Which I don't think had been done in a film. The closest thing was in Alien.
Lofficier: I'm not a scientist, but did you check if she really could have survived for such a long time, considering what she was experiencing, the decompression, the air rushing through, etc?
Cameron: It's artistic license really. I've studied enough physics in college, that if I had sat down and worked it out, I probably could have computed the amount of volume of air that the room should have in relation to the size of the airlock...
Hurd: Besides, you never see the room in its entirety. It could be huge!
Cameron: Or you could presuppose that all the air throughout the entire ship is being drawn out. Obviously, she was able to override the interior doors so that they would not automatically close when the outer doors opened, which is how a real airlock would have to work, safely. But one could assume that they had the capability of doing that so that they could load things in and out through that door, straight into the ship.
But I think it's very unlikely, not that she could breathe, because for the length of time that the air was going out, she could breathe, but that she could hold on. The wind velocity would probably be somewhere around 300 to 350 miles per hour.
http://www.lofficier.com/cameron.htm