Just watch all of them so you can compare. I think it's better to watch them in non-chronological order (original trilogy then the prequels) because the originals establish a lot of things that the prequels don't (like what the force is).
I'd do the original trilogy first, simply because if you start with Episodes 1, 2 and 3- you may never make it to the good ones. (THEN after you've seen them all, go back and watch them in chronological order!) Happy watching!
I like the idea of doing IV, V, I, II, III, VI. The first two of the original films creates the background and establishes a big surprise twist regarding Vader's identity, then you get a flashback of the prequels, then the conclusion with ROTJ.