I think you're right, I think that Pullman does deal in archetypes, but I'm also pretty sure that Rowling does too - and they are VERY familiar. Reluctant hero, his loyal friend, boffiny friend, spoilt brat enemy, evil enemy, unfair teacher, merliny/obi-wannish headmaster, etc...
Something that seperates them for me is that there is a lot of repetition in the Potter. If everyone things that Harry's being arrogant when he isn't ONE MORE TIME I think I'm going to push a cotton-bud so far up my ear I kill my brain. Perhaps its in the nature of a series of books which are tied to a location and the structure of a school year, I don't know. But the unfolding of the plot feels more mechanical than HDM. It feels like there is less at stake somehow.
I also prefered the unfamiliar quality of Pullman's world. Rowling has cross fertilized a fun, ripping school yarn with familiar fantasy and traditional magical elements. Pullman has made something darker, more unfamiliar, but equally as well thought out. Give me an alethiometer over an invisibility cloak (what... a cloak that... makes you invisible?Ye gads!) every time.
I'm totally cool with you liking Potter, because many of my friends do, and its obviously just a matter of taste. Its just that so far (now more than half way through book 4) I'm still prefering Pullman. But I guess the very fact that I'm reading Potter still suggests that I'm enjoying them, and I am.
Cheers