Planetary was a good comic and an interesting experiment in compression, but not an as good a comic as it could have been in my opinion. I think the effort to compress the arcs to such an extent as to make it an 'episodic' comic book ultimately hindered the book, as much as holding another book to a rigidly decompressed writing paradigm can be a hindrance. In any fiction there are elements of a story more deserving of energy and focus than others and certain ideas that require more panels of exploration to be adequately translated into written word. The best way to write a book is to identify these disparate parts, prioritise them, and convey them to the reader in such a manner as is reasonable given their relative importance and complexity. Sometimes that'll mean several issues, and sometimes less than one, but the important thing is that one makes sacrifices with regards to his or her style instead of at the cost of the story. Simply put, compression introduced challenges to Planetary's pacing, plot, and (primarily as it pertains to the antagonists) character development. Not severe enough to make these anything short of an interesting, enjoyable book, but enough limit it from being the book it had the potential to be.
And I don't see the point in comparing Planetary to Watchmen any more than I see the value in comparing Blood Meridian to Pale Fire. There's nothing impossible in doing so, they're both books, share some fundamental elements, and belong to a literary tradition, but they're nevertheless wholly distinct works, by completely different creators, and intended to do different things in different ways. That they are both intelligent books does not in itself necessitate or rationalise their comparison. For the record, I prefer Watchmen for various subjective reasons of my own, but that's all most of these comparisons will ever really come down to. Planetary still hangs out somewhere in my top five, I'm just not entirely sure where.