Indeed, Lost Cause II has its charms, but you can't beat the "first love" (as one my coleague usually likes to point out). And besides, I miss the original Princess Leer. On the other hand, Ham Slomo has headed into much finnier direction, which is good. I also didn't like Lube being turned from italian-american (correct me if I'm wrong) to texas cowboy.
I agree with you, Part 9 will be really hard to beat!
All in all, it is impossible for me to choose between the two series. Best of luck with the future work, I can't wait to hear what awaits us!!