Stone would certainly make for a more attractive design. With less concern for costs, a poured slab with a brick wall and faux stone exterior would be super sleek, and stand forever as a gorgeous monumental success.
I have a hard time justifying to myself any expensive projects that don't eventually pay me back with higher rewards or savings. Building a $1000 planter just to grow $1 tomatoes feels like a waste to me.
Good luck! I hope you choose something that brings happiness.