My p90a build is coming along slowly, but progressing none the less. Ive rebuilt my hydraulic lifters, replaced the springs inside with new ones, and now im getting started on the head. I would go p90a, because the valve guides are stainless steel and last a lot longer, if not forever. If you are concerned, check out this thread to see what you are getting into with hyrdaulic lifters. http://hybridz.org/phpBB2/viewtopic.php?t=26904&highlight=hydraulic+lifters
The p90a is truely a great head, and the last inline L series head that Nissan marketed. I have one in perfect condition. People say you cannot run a performance cam on these heads due to the hydraulic lifters, but i am certain that it is possible, after researching and spending time with the head and its components. Good luck.