There's no universe in which Sané is more complete than Sterling at present.
Sané is very good at his main job, beating defenders one on one and crossing from the byline. He's shown a lot of potential in other areas, enough to make us all think he's got the potential to be a world beater and pretty complete forward, but if you're talking about what you can expect each week, thats Sané's big strength right now. His decision making and passing on the break is pretty temperamental, he often goes an entire game without getting into a shooting position, he is still pretty inconsistent in general and there's definitely a pattern of playing much better at home than away.
He is, basically, a developing young player. An exceptional young player, but still a long way from the finished article. This isn't criticism, it's just how young talented wingers are -Cristiano Ronaldo was the same at the rags for many years and a lot of people have made the comparison over the season to when he was a similar age.
Sterling on the other hand is much more involved in the general creative play, and breaking down defences, he has his role as a wide man but also his central role, dropping markers and finding space in the box or drifting into midfield. Pep has been able to play him up front next to Jesus in a 2, on his own as a false 9, as an attacking midfielder like on Sunday in Silva's position, as well as his bread and butter on the wing.
The reason he can do that with Sterling is because his game is very well rounded.
There's a good argument to be had about Sané having more potential - he certainly has world class potential and the world at his feet. You could definitely say that if he fulfils all that potential he might be a more complete player than Sterling will end up - but right now, there's no question Sterling's play has a lot more dimensions to it.