This probably isn't reliable as its from the devs of towermadness. I noticed one flaw already stardefense will have downloadable maps in the future but for pay, not free like tower madness
SD looks more polished and complete and the globe thingy is unique TM has a unique quirky charm to it, and the open field leads to more strategy i think ... both are 3D and if u include sweetwater defense, thats 3 3D tower games in the last week!
Playing Star Defense has actually made me consider TM and SweetWater oddly enough...I might just pick up Elemental TD