I think this really boils down to 4 things:
Looks, handling, 0-60 times and exclusivity
This is what we know from the 3 cars and just a little from what we've been told on Evora.
Cayman S
Base Price: $60,200 and $63,620 w/PDK
(-$14,795 and -$11,375 compared to Evora)
HP: 320
(+44 HP compared to Evora)
0-60 MPH: 4.9 seconds, 4.8 seconds w/PDK and 4.6 seconds w/PDK Sport
(0, -.1 and -.3 seconds compared to Evora)
Top Speed: 172 MPH
(+10 MPH compared to Evora)
26 MPG 
(-6 MPG compared to Evora)
Curb Wieght: 2976 and 3031 lbs. w/PDK
(-70 and -15 lbs. compared to Evora)
911 Carrera
Base Price: $76,300 and $80,380 w/PDK
(+$1305 and +$5385 compared to Evora)
HP: 345
(+69 HP compared to Evora)
0-60 MPH: 4.7 seconds and 4.5 seconds w/PDK
(-.2 and -.4 seconds compared to Evora)
Top Speed: 180 MPH
(+18 MPH compared to Evora)
25 MPG (-7 MPG compared to Evora)
Curb Weight: 3075 and 3164 lbs. w/PDK
(+29 and +118 lbs w/PDK)
Evora
Base Price: $74,995 -but could come in a little lower as told by Lotus
HP: 276 -as listed
0-60 MPH: 4.9 -as listed
Top Speed: 162 -as listed
32 MPG -as listed
Curb Weight: 3046 -as listed