I think Britton will be top 20 player, maybe Janowicz as well.
Other top 100 at best.
Britton has won only one match on futures circuit, I believe. For a player of his age (born 1991), that's very little.
He hasn't played many either to be honest
I don't know why but it's like that
Among the Americans from that generation, Domijan is the most regular on the adults circuit.
As for the question, I'm not really able to answer, as I've not seen them playing : according to the results, I would rather say Janowicz.
Raonic and Pospisil met recently : it was a very tight match, and Raonic is a bit younger, I can't say