Your chest strength on the smith and on the bench are the same. It's not like your chest takes one look at the free-weight bench and decides it looks scary and wimps out. They dont appear the same if you judge by poundage on the plates, but all things added up, force is force they are the same. Just that on the bench, you have to deal with balancing the weight as well as pushing it up. Bottom line is: as long as you use a heavy enough weight to struggle, that's all that matters. As for muscle building effectiveness, both are effective. Smith allows you to push the maximum weight you can push without worrying about injury, while the bench will help you build the neighboring muscles more effectively. Use both to get maximum benefits.