You can do it either way. What should decide is how much emphasis you want the etching to have.
If you etch on the inside of the window it will be less pronounced when backlit and viewed from the outside. This is the best way to go if you don't want the etching to steal attention from any other mods inside your case.
But if you have an etching design that will take center stage on your mods, you probably want it to be on the outside of the window. An etching done this way will command most of the attention when viewed, while whatever is behind the glass (what's inside your case) will fade into the background.
If you're working with an image, you can play around before you decide. Do your etching, tape it to the window hole cut in your case with the image facing in and then facing out, and judge for yourself which gives you the look you want.
i'm going to use a dremel to etch the AMD logo on my window....I'm using an antec case, so the door latch obscures most of the interesting stuff already....and its not like I have any other mods on the inside, so I think i'll be doing it on the outside.
I must say, that dremel has been my friend throughout all of this...heh. I've done everything with it except drill the holes.
Well, maybe I should order some cold cathodes soon, so I can light this baby up, w00t.
oh, I'm using the tiny *** one, because it came with and I dont feel like spending money on another one....i've sunk enough money on bits for this project, hehe. The etching is taking a while, due to the large surface area that I must etch out, but its going well.....I finished the little thingy in the bottom left corner, which took quite a while to do....its very slow going....if you have something similar with a large surface area, i would advise you get a larger bit....but I still got the large part to finish, which should take maybe an hour, heh....
I did have the opprotunity before I started etching to attach the window (bolts) and put it on my case...and i must say, it is beautiful. I will be sure to post pics when I finish...and then some more when I get some lights installed, heh.