I nearly bought a 911 as I needed the practicality of 4 seats to squeeze in if necessary and similar size boot/trunk rather than the less practical R8.
In the Uk I could only buy a 2012 911 3.8s with around 40k miles vs a much newer 2015 i8 with only 17k miles in much better condition.
What put me off was the running costs of the 911. Everything is so much more expensive so factor that in the depreciation which now seems to have steadied out. Also depends if a daily driver and how many miles you do.
The 911 might hold its value a little better but as I said, factor in the thousands a year running it.
911 vs i8:
High uk road tax vs free
Sub 20 MPG vs 40-80 Mpg (my 8 miles each way commute is elec mainly)
High Porsche servicing vs my local BMW specialist averaging £300-400.
My Uk insurance much cheaper.
Uk used prices now good value and steady.
Performance figures of all cars you mentioned very similar.
So far, it’s the cheapest running cost car I’ve ever owned!