None of us can tell you which is best for your needs since we don't know you or your children. I have a 21 foot Class C with sofa, dinette, kitchen, dry bath, storage closet and overhead bunk. Some think its tiny and can;t imagine anyone living in it while others that RV like I do think it's way too big. It can easily sleep 3 adults. For the way I RV, it's an almost perfect RV.
You mentioned the spontaneity and shorter duration trips. That indicates you may be wanting to take off without much planning or preparation. With my small RV, it's often used as a second vehicle which means it's always in ready mode.
You and the kids need to hit up a larger RV show and sit in various models from various manufactures; Not to think about buying one, but to gauge the feeling you all get when inside different manufacture's models of similar sizes to what you're considering. You will quickly figure out if a U or L shape dinette is sufficient or if a sofa is really needed. You'll see if a extra jump seat is a waste or an added feature. I known people who walked away wondering why anyone would want a slide considering how on some models it actually reduces space.
So take the kids and hang out in some just to see. If still not sure, rent each for a weekend and try it out. Renting is a cheap way to focus in on size and amenities compared to buying the wrong one.