Actually, unless you are using a LOT of acid, I'd recommend getting the 15% . . . as long as it's 15% hydrochloric, and not 15% sulfuric.

31% hydrochloric (commercial muriatic acid) fumes badly, and even small leaks of fumes can cause serious damage to electrical wiring and exposed steel. That's why I recommend against storing it inside. 15% hydrochloric only fumes a little, and 10% doesn't fume. There are some long stories behind this, but trust me when I say that the savings on acid is far smaller than the potential for damage if you are careless about storing full strength muriatic.