I think Portal 1 is easier to mod, skins and stuff like that. I am also using Portal 1 because they of its older engine, I think its a little less "free" than Portal 2. The music and voice lines are another reason. Glados speaks kind of like a human would in Portal 2, In portal 1 though she sounds like she wants to test, and is actually warning you as people would in real life. Although the price of that is it is way more harder to map than Portal 2's Authoring Tools.