I was actually debating this for a while too! I didn't pick riolu because i thought that would fit iida better considering the fighting/steel typing and how lucario/riolu is a speedy pokemon. Also i factored in color schemes as well riolu is blue and black, while chespin is green. The big reason I chose chespin was generally looks, fighting typing, color and the fact that its a starter type and I had given Bakugo a fire starter and I thought it fit the tri evolution rivalry much better haha. Since your rival always picks the pokemon with a better type advantage.