Well, first of all, remember that you're going to be a new grad- sometimes it can be overwhelming, no matter WHAT unit you work on. You have so much to learn!
What I'd probably be focusing on now is the staff and administration- are you choosing between units at only this facility, or are you considering others? If you are deciding between NICU and PICU in just this one children's hospital, take a long, hard look at the staff. How helpful were they to you? Did they answer your questions? Did they go out of their way to teach you new things? Did you feel nurtured by them, or pressured by them? These will be essential factors during your first year! These people will be your mentors, and you need to feel comfortable with them so that you are able to ask questions and not fear being ridiculed or talked down to.
There is a significant difference between the PICU and the NICU- PICU does get a larger variety of cases. They get accidents/traumas, we do not. NICU's do get somewhat of a "standard" array of illnessess, but that scope is not a narrow one, and it has provided me with more than ample learning opportunities. Your chances for a "typical" NICU patient are much larger working for a smaller, non-regional NICU than they are at a children's hospital: one thing about children's hospitals- those kids are SICK. For many, they get transported in from all over to get care there- out of state, out of the country, because there is no care available for them in their hometown. Babies aren't BORN in a children's hospital, they're BROUGHT. This means that these are the sickest of the sick, and that generally holds true to the PICU and every other unit. If you're looking for variety in the NICU, a regional transport hospital or children's hospital is probably the way to go.
I've heard people say they felt the NICU was boring, and I can't imagine that. To me, it is FASCINATING.
Because the PICU deals with a larger variety of cases, I'd suggest that your priority will be with new-grad orientation and preceptorship. You need to find out what this hospital offers in the way of continuing education, lecture series, inservices, etc.
This should also be a priority in ANY new job that you look for in nursing, but it is particularly relevant to PICU/NICU because at some point, you will be expected to work and function as a competent nurse, and these kids are VERY critical.
The PICU nurses I've met are extremely knowledgeable, and I admire them, but I have no interest in working in PICU personally. If I did, I know that my experience in the NICU would help, but I don't think I'll ever choose that road.
Good luck! Let us know what you decide.