I do understand you dilema as i was in a similar boat but maybe not so high stakes!
I nearly bought a used vantage (Two year old ones in UK for £60k similar to an I8) but ended up buying a Jaguar XK-R mainly due to having small back seats.
This was my pride and joy for about 3 years but it just sat in my garage and spent more time polishing it than driving.
I was running two cars and the i8 was my answer. So far, it is doing just that and i'm enjoying driving it more than the loud roar of the jag or fuel bill.
I think it's all about how the car makes you feel when driving it and it's not really about the car it's self. I like more unique cars that stand out.
As you sound like your Aston also will sit for months in the garage, the i8 will give you that feeling and without the huge running costs so my advice is 'go for it', keep the rest of the cash in the bank and you can always go back to Aston in a year or two!
I think the I8 has a younger man's image like the Aston is the Gentleman's.
Sort of Wayne Rooney footballer Vs Daniel Craig James Bond!
