Good timing of you to make this! Finished Portal for the millionth time and have now started on 2 for the first time. Not sure if I prefer it (just after wheatley takes over) but it's definitely on par with it so far, and the stages are WAY more challenging imo.
I never actually owned Portal; so I never finished it, but I played it on a friend's computer. I wish I had played through it before starting Portal 2, though