With offensive missiles you choose your target lock on and launch end of story. With missile defense you need to be able to track a small moving target and then shoot it down. The tracking it takes is much more complicated and its because missile defense system are so complicated so far the only true balistic missile defense systems on the market today are the Arrow & shorter range Patriot and SM3 whilst Iran has not developed anything so far which involves complicated tracking technology which they just don't have access to. Its going to take Iran years to develop any kind of practical missile defense which would and by that time other countries will probably be ahead of Iran in terms of offensive and defensive missile systems.Israel has already developed ICBMS that can hit virtually every country in the world and submarine & aircraft launched cruise missiles and the Arrow 3 under development has the capability to shoot down a missile miles away from Israeli territory and it can shoot down Iranian sattelittes. The S-300 is also not a missile defense system its anti aircraft ( designed to shoot down planes not missiles ) so even that is not a true missile defense system. Iran has allot of catching up to do.
Bomb in Lebanon targets bus heading to Syria, no casualties (Reuters)
from the article: Iran official: Russia obligated to complete S-300 sale