Phenolic or Metal Piston?

My Dad's 2000 XJ needs new calipers and I have two choices at the parts store. From experience here which would you select?

Late edit: just re-read the topic and realised I answered the wrong question. Gah.

And I'm with old_man on metal instead of plastic.

I'm old school. I like metal instead of plastic.

I've never had caliper problems with either type on an XJ, but the phenolics do chip easily if you are careless when replacing pads. If you have regular problems with seizure, they might be preferable because they don't rust. Otherwise, metal is more robust, and probably a safer bet if you don't know who will be doing your next brake job.