http://www.pets.ca/forum/showthread.php?t=76896
The recent thread I linked above in and of itself will give you an idea of the complexity of your question. Within the thread I posted a list of articles you might also find to be interesting reading on the age to spay or neuter. There certainly is not a "one size fits all" answer. You are quite correct, research shows neutering prevents some health concerns but is associated with higher counts of other health concerns.
One thing you can definitely say to any who rudely disparage an operation you had no control over, it did save your pup's life. Most shelters and rescues demand the critter be neutered before leaving their facility. What would have happened to your pup otherwise? Illness or death on the streets?