Corey,
I have a 20g reef setup. If all you are planning on doing is fish, I agree that you should go with the live sand and a back filter along with often watch changes. Live rock will not survive without enough wattage ( 3w per gal. ) However, if you plan on spending the money on a light, go with the live rock/ soft corals as well. All I have on my tank for filtration is a back filter with a bio wheel and a powerhead. I am going to be switching my tank over to the berlin system ( live rock and a skimmer ) here soon. So if your doing fish only, you need nothing fancy. Just a back filter and often water changes. However, if you choose to do a mini reef I would add a better filter or go with the berlin system. Just rember dont belive everything you hear at your local fish store. There going to try and sell you everything you dont really need. Just rember in a tank that small the key is frequent water changes.