well, just a few facts I've found...
Hoop skirts:
+ lighter & cooler
+ they give your skirt more volume with less bulk (in the Victorian era -before hoop skirts- ladies were known to wear up to 8 petticoats.. that's not including the under petticoat and over petticoat - though they still wore the under and over petticoats with the hoop skirt)
- it's a bit tough to sit down - impossible in some (luckily mine only has one hoop at the very bottom, so it only sticks out a bit because of the legs of the chair)
- it's a bit tough to get through doorways (though in older houses -like the 112year old Queen Ann Victorian my family and I just moved into, the doorways are extra wide because of the fashions of that time)
Petticoats:
+ they come in various lengths and thicknesses
+ they are prettier
+ you can sit in them more easily
- they are heavier and hotter than a hoop skirt
- you may have to wear up to 8 of them to equal the volume of one hoop skirt
- some are quite itchy.. though this can be solved by wearing a slip underneath it
also hoop skirts come in various shapes while petticoats are just.. well.. round. Here are some examples:








and some good websites with information and/or pictures:
http://www.victoriana.com/Victorian-Fashion/crinoline.htm
http://www.antiquedress.com/gallerymuseum22anew.htm
http://www.schenz.com/ts_costumes.html
Hope I've been of some help.. and good luck with whichever you choose.