Ok We are in a big debate, first off we are in a off road racing series here in the northwest. lightwieght jeeps with big horsepower, Yea you got it! A blast! ok, our debate is will the 6 inch rod be more durible than the 5.7? or the other way around, or dose it really matter with the after market rotating assemby's you can get? I have heard a 6 inch rod is better for higher rpm's is this true? we turn between 6 and 7 most of the time and I want something thats going to last a few seasons, I have out grown my 350, My foot is always on the floor! we are ruled to be carbed and no nitrious, other than that, fair game! but we all agree over 500 hp is over kill and your under your rig more than in it, hope to hear from you soon!  

Length won't make the rod more durable, that's really controlled more by the material and design of the rod.

That being said, you're better off going with the 5.7" rod so you don't have problems with having to get pistons where the wrist pin intrudes on the rings.