For my mouth, these 2 dishes are really good, BUT I tend to say pho. Now there is a point I don't really understand (in France, I don't know about others countries), especially with Asians :
A new Asian restaurant has opened. So one say :
"Do you know the restaurant ?"
"Hmm... no, is it good ? What about their pho ?"
It mostly starts with these 2 sentences. 2 possible cases now :
Their pho is good : "Cool, I'll go to this restaurant next week-end and try their PHO."
Their pho isn't good : "I won't go."
Well, why asking if the pho is good. There are lots of others dishes. I've never heard someone ask if their papaya was good.
Of course, it depends about which specialities we consider but really, why pho.
So, my conclusion goes to : Pho is more popular.
However, popularity depends on the country. I was in Vietnam years ago and I had pho for breakfast, lunch, break, dinner... I was in Thailand after Vietnam, there were this old woman that sell papaya salad. Of course... we bought all her boxes.
Ok, break time, I'll get something to satisfy my tummy.
Edit : I wonder how you guys eat the papaya ? (Does it exist any other way to eat it ?) I like it with bread and lots of spice, like a papaya sandwich