I really think it depends on our opponent and who our other defender is. If we're playing against a team with a high line and have Adriano or Montoya playing, we want Tello to exploit that space and make up in attack what our fullbacks aren't offering. If we're playing against a team with a lethal attack and we have a more offensive fullback, then yes, Pedro is the better choice. But I feel neither players has reached their full potential yet, and if Tello gets time to develop I truly think he could become an excellent player. Predictable sure, but so is someone like Robben, and yet he seems to be doing just fine.
The thing with Robben is that while often frustrating, he can produce moments of brilliance despite his flaws. I'd compare Deulofeu last season to Robben, unbelievably talented but comes with a huge bag of unwanted traits to go along with it (although he's shaking them off). Tello, while he can exploit his pace, isn't a player with the innate footballing talent to be an elite player. The way he plays against high lines is very useful, as seen today and sometimes in the past. However he needs the game to suit him to perform, he can't perform in every situation we need him to.
I'd pick Pedro every single day of the week. Even if he didn't show it today, he has the ability to exploit high lines as well (his hat trick vs Rayo as proof of that) and he's a much more fully developed player (he's gained the confidence to go for goal more often now too).
Let's not detract from Tello's performance today though. He did well and is taking a step towards getting back to form. We could have good use for him as the season goes on and we need rotation, Tata seems to be more eager to give him minutes now as well. The thing Tello has in his favor is his shooting. Granted he can't do it in every single situation, but he should play to that as much as possible and take it as his ace in the hole for the future. We could use that while we still have him, and if he leaves sometime down the road he could provide it for a club in need of a pacy wing goalscorer as a weapon X.
