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I’d say go with the breed where you can find the better, closer, breeder. 
I’m sure others will chime in with more details on breed traits. I suspect both will have similar issues with housebreaking and/or marking, just like all dogs do. I think some people have a harder time housebreaking small dogs; not sure exactly why that is - whether it’s because they are small, and low to the ground, or for other reasons.
My dog’s breeder also has two papillons; both well-bred for conformation. The older one is nice - sweet but a little shy…apparently too big for conformation. The younger one is a lunatic and I’d probably give the dog away if I were mine. She is SOOOO annoying. She spins and spins and licks you until you’re soaking wet. It’s possible that she needs more exercise and a job - she is only 3 or so. But I think she’s the most neurotic thing I’ve ever seen. I have no idea how my friend trims her nails or cuts her hair, because she never stops moving.
So…as I said, others can give impressions of breed traits - I do know that papillons are quite athletic despite their small size, and often compete in agility. Not sure about poms. But definitely find a good breeder and evaluate temperament.[/QUOTE]
Several reasons shrimpy dogs tend to be harder to housebreak, one that they are given comparatively to their size too much room to roam before they are reliably housebroken.
Let a big dog roam a room or three and keep an eye on it and you can get it outside before it has an accident.
The same room to roam for a little bitty dog, it is a long ways to some far off corner the dog may decide is a good place, away from the action, for a designated bathroom.
Also, it is a bit easier to notice a big dog sniffing around and looking for a place to go than a little dog somewhere in a far off corner.
We had plenty of toy dogs and didn’t have any problem, but we trained/confined them carefully so they didn’t.
As far as those two breeds, I agree, in general, pomeranians tend to have a more steady temperament, papillons may be more active, energetic, busy minded dogs.
It depends on the people and the individual dog which would fit best, hard to say without knowing them.