PX100s are open (with flat foam similar to that on sony headphones), PX200s are supraaural and sealed. They both fold. The best bet if you can't find a retailer where you can try them on is the PX100s (or other phones, like the KOSS portapros), because the PX200s aren't always a good fit (supraaural means they go over your ear, so unless you can get the pleather-ish material to seal over the grooves of your ear, they won't isolate well).
www.head-fi.org www.headphone.com has good pics, as does this review:
http://www.dansdata.com/3senns.htm 
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