:o Here we go again.
I assume that you are referring to the 17's since those are the only two that you can really compare.
Bigfoot is taller, wider, has stove with oven, smaller bathroom and smaller blackwater tank (8 gallons) - unless that has changed and they haven't updated their website. I believe it can sleep 4 adults. However, I do not believe it has the capability of having a dinette and a bed made up at the same time.
Casita is narrower, larger bathroom, larger black water tank (15 gallons), has two floorplans that allow sleeping for two adults and a dinette both up. Also has a unit that has a king sized bed or narrow twins or dinette seating for a group (not at the same time). This unit could be used with a double bed and a dinette in front of the bed.
Those are the differences right off the top of my head.
You decide which features are important to you and then you can choose.