As has been mentioned, Kerry's success was mostly in World Class Championship Wrestling, which his old man owned. However, to be fair, that doesn't mean that the guy was automatically a shitty worker. Kerry was very good back in his prime, but what ultimately did him in was his own self-destructive behavior. Rather than intervene on his son's behalf, Fritz was content to look the other way as long as they were drawing huge houses every single night. If Fritz had stepped in and put his son's health first, there might've been a chance for Kerry to go onto other things. But, cest la vie.
I've always respected Rey's athleticism and his willingness to take the big risks. However, being 5'5" and about 165 pounds doesn't really leave him a whole lot of choice. However, Rey's best matches have always been against men of somewhat equal physical size such as with Dean Malenko, Eddie Guerrero or any number of the luchadores that were running around WCW back in the mid to late 1990s.
Both of these guys have two of the suckiest finishers in the history of the business. The 619 is flat out lame and that splash Rey does off the top rope now...it just doesn't work with someone of his size. Von Erich's primary move was a spinning punch called the Discus Punch. Both guys were essentially handed World Championships as part of a type of tribute/explotation of the death of someone close to them. Kerry lost the strap two and a half weeks after he won it from Flair and many never bought Mysterio as the World Heavyweight Champion. I remember a match he had with the Great Khali while he was champ and he was simply obliterated. Rey's size is a hindurance because there are simply some situations that fans aren't going to suspend disbelief for.
I think it'll be fairly close, but I think Von Erich's strength with simply be too much in the end for Rey. I think it could be a great match, but I'm giving the nod to Von Erich.