It also depends on how compressed the source is. If you compare a 1.5 GB 720p movie with a 2.5 GB 1080p you'll see a bigger difference than less compressed sources of 720p and 1080p. The size of the screen you're watching it on is also important.
What size is your smart TV? If it is 40" or less, with 20/20 vision you probably won't see a difference at normal viewing distances (±2.4m). Try out this awesome calculator to work out optimum viewing distance : screen size : resolution http://www.rtings.com/tv/reviews/by-size/size-to-distance-relationship
Good question, I have two tv's. One 720p 42" Plasma and the other a 1080 32" LCD. Though some stuff might look awesome in full HD, I really don't gives a rats ass.