[Brightcolours]

Thanks for showing this equivalence I didn't think of.

However a practical approach: I have Tokina 16-28 and canon 10-18.

It is obvious which combo is lighter and 10mm f8 has more DOF than 16mm f8 since lighting is the same and angle is the same, so I prefer the crop version.

Of course if I needed shallow DOF and low light performance I would use the Tokina 16-28.

Perspective at 10mm should be different from perspective at 16mm however I didn't notice this in real life practice.

Now being more and more practical, ultrawides isn't my style and I rarely use any of them but I am too anxious to carry a bag without an ultrawide inside "in case I need it"

Lighting will the the same for 16mm f13 (FF) and 10mm f8 (APS-C) when you set APS-C to for instance ISO 200 and FF to ISO 500 or so. 

 

Why would there be a different perspective for 16mm on FF and 10mm on APS-C? There actually is no perspective difference. Same subject distance and same FOV.

 

APS-C has the price and weight advantage. FF the shallow DOF possibility advantage.