There are nothing but pros of neutering your Papillon. (Best at 6-8 months of age)
The only "con" I can think of is the actual operation. there is always a risk involved putting a dog (like people) to sleep.
But the "pros" are numerous:
You would help preventing several problems that your Papillon may get in an un-neutered stage.
Females may develop tumors on their mammary glands if not spayed.
There also found plenty of cancer in the sex organs of our canine friends (uterus, testicles),which are removed when neutered.
There is less of a mess: Females can get quite messy (bloody discharge) while in heat.
Boys, when neutered in time will most likely never think about marking their, or other's territory.
A neutered Papillon will be a calmer, "more in peace with himself", dog. He/She is not driven by a steady flow of sex hormones, therefore:
No roaming, more attention to the owner to please him or her.
A neutered Papillon will have a fuller hair-coat.
There is one slight disadvantage I can think of:
The Papillon may get a little less active and because of that a strict diet is recommended to prevent weight gain.