There was/is an article that was done about this exact subject and the differences are minute. I wish I could find it. Anyway, my "opinion" is that people make much too big of a deal about this subject. The side loading vs. time at TDC, etc.. the difference is so tiny it is really not worth the effort. While I agree in most cases the longer rod will have a lighter piston BUT, you will usually have a heavier rod (since it's longer). Like 99% of hot rodding it is all give and take. Buy what works best for your application and is priced best.
I have built 383's with 5.565" (stock length 400 rods), 5.7" and 6" rods, there really is no difference when it comes to performance.
This is one of those topics that will probably never really be agreed on. Build it either way and it will probably run about the same, so there is no real risk or reward.