Ice Cream truck biz, should I LLC, SCorp, Ccorp or none?

My sister and I are going to own an ice cream truck to operate this summer (and future summers). I guess we need an EIN for tax purposes. Should we become an LLC or Scorp or something? It would be nice to have my personal possessions separate in the event we should be sued for something and I'm not sure what would be best tax wise. It's pretty basic; we;ll sell ice cream, keep track of our own profits and losses and open a checking account that is just for the business. Any advice before I get this EIn would be appreciated. Please keep in mind it is myself AND my sister, equal partners.