Tough questions, I think if he gets a well bred dog from either breed and socializes it, trains it and spends lots of quality (working, competing, playing) time with it you can't go wrong. I think how well his experience goes with the either breed will depend mostly on how much time he is willing to invest.
That said....
I have owned three Rotties and only been around some GSDs but never owned one.
Rotties are usually bigger, more laid back (on average), and are more defensive driven than prey driven like the GSD.
I think on
average you see more cancer in Rotties but more allergies in GSDs.
I think on average GSDs have better work ethic and deal with heat better.
Rotties are better swimmers and I think on average like water better.
I think GSDs are better at fetch than Rotties on Average.
Both do well in the cold but I don't think either should be an outside dog because they love their people to much.
From what I have seen, in Schutzhund training I have watched a few GSDs can get frustrated more with handlers and travel up the leash. I have never seen a Rottie do that.
Rotties growl more even when happy. I had a Rottie that had different growls for every occasion. I haven't seen a GSD do that.
When I think about it GSDs vary a lot also. Just in one breed you have Working Lines and Show lines. In within those you have West German, East German, Czech etc.... in the working lines and divisions in the Show world with American GSDs being very different than West German show dogs.
Lots and Lots of variables. Perhaps a more specific question? DanL knows tons about GSDs and RedyreRottweilers knows tons about Rotties. Maybe ask your friend to log on and ask more direct questions.
For GSDs I also highly recommend going to
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