Aha, yes 200+ miles a week in the uk I would say classes a lot. Thats probably a commute to the local shops for you in America.I was wondering how in the world 200 miles a week is a lot of driving, then I noticed your location. I still forget how different our countries are in that regard.
On to the main point: I think you have pretty much nailed the differences here. Lotus depreciation, in my experience, is certainly present for about 5 years and then stabilizes. My Elise hit its bottom value around then and stayed stable until it was about 15 years old, then the value started climbing again. I haven't owned an Evora, so I can't really speak to that depreciation, but they seem to have done fairly well from what I have seen in the second-hand market. Porsches are available in FAR greater quantities so their results are more known, and their GT cars are, naturally, much flatter on the depreciation curve. A better match-up to the Emira is the 718 Cayman GTS 4.0. The GT4 gets more into the track-weapon range and gives up quite a bit of comfort. I have driven one and felt it too harsh for normal driving.
Emiras are surprisingly roomy inside. I found the 718 to feel much more snug. The boot is just part of the overall cargo area.
The 718 is a better "car" in terms of livability -- except, again, I think you're looking at the wrong 718. The GT4 is rather harsh. Everyday Driver's resident Porschephile and 718 GTS 4.0 owner said he preferred the Emira but agreed they're also not so different.
I would say drive them both and then buy the one that you can't stop thinking about.
My issue with the GTS 4.0 is that it doesn't have that fast car look or a wow factor that the Emira has. Whereas the GT4 has that extra styling details that make you turn your head when one drives by. I have a test drive booked with Lotus in a weeks time and I am looking to schedule a drive with a Porsche soon.
Thank you for your advice with the depreciation.