A Passer Angel is very aggressive, especially as it gets larger. One the other hand the Passer doesn't get as large as the Blue Ring, and for that matter most of the Larger Angels. The question is what size tank will it be going into and what other fish do you plan on keeping the Angel with? If it very aggressive fish, like Grouper, Trigger and such then a Passer will probably have a better chance of holding its own with them. As for which is more hardy, well both are fairly hardy but I would say the Passer is more hardy of the two. You'll also find the Passer has more personality, as it will be out front and even flollowyou around.