I don’t claim to be an expert on either, but better goal scoring ability? Lozano scored more than double the goals Ziyech did last year, in less games, and is 3 years younger.
There's also the fact that Ziyech would be wasted on the wing. He would have to play central to be effective.
If Silva comes, we're likely to use a 3-4-3 or a 4-3-3/4-2-3-1 kind of hybrid.
I'd imagine we'd have Gueye and another in central midfield. Ideally, that 'other' will be someone like the lad from Lille who can get about the pitch and basically play the role Doucoure did at Watford or N'Diaye did at Hull under Silva.
We have Gylfi for that role just ahead of them, and also Davies/Klassen. It'd be nice to get some more options but it's not a priority I don't think.
So the winger for me has to be a real potential star - and Lozano fits that bill. Creative and can play on either side, perfect to interchange with Walcott.
Ship Bolasie off and Lookman is on hand to play there as well. Lookman also now has the added experience of playing in a front 3 or 2 at Leipzig.