Full Face, in my experience, most shops (in fact ive NEVER had a complaint) are cool with leaving your helmet on if you can't be arsed to take it off.
Get something decent tho, doesnt matter how ****ty your moped, slamming your head into a curb at 30mph is going to need some protection.
I have one of these.
http://www.lings.com/lingscentralapp_core.lassoapp?pg=nav&prodview=fc6043f5a3ae9988§num=c7c38973f3fe9736&catnum=35c2ff123a2df76d&-session=Lings: