There are critical flaws with watching the prequels and original movies in either order. Watching them from 1-6, you lose the surprise that Vader is Luke’s father. Watching them the other way lacks suspense in other ways.
To fix these issues, Rod Hilton explains his solution: The Machete order and the reasons behind it, which I reckon are brilliant. If you want to psyce yourself up for The Force Awakens by watching the two trilogies, what better way than in this order?