Stephens is a good player but lacks mentally. Theoretically, she can go toe to toe with Williams on a good day, but the desire that Williams has is sadly missing from Stephens. Williams comes through in a clutch. Stephens disappears.
Keys is a bit of a beampole server but can beat anyone on her day. Suspect her attitude is better than that of Stephens, but without the consistent groundies. On the whole though, she's probably more likely to win a major than Stephens. Keys is the sort of player who could be a non-factor for ages then suddenly pop up and win Wimbledon just by holding serve.
Right now, I would say Keys is top ten material, Stephens top twenty. I'm guessing America wants better?
Well, the good news is, once Cici Bellis kicks her father into touch and actually starts playing more tournaments and building her physique, she should be better than both. Not in Williams' league, but certainly America's next major champion; an all court baseliner with solid hands and a steely attitude. Sort it out, Cici.