It's all down to personal choice, and obviously I'm biased. However, RE: interior space, I'm 6'-4", 205lb and have waaaaay more space in the 12c than the GT. I can also drive with a helmet on in the 12c without having to adjust the seat to the Italian ape driving position to get necessary headroom. I can also put a big Igloo cooler or two cases of wine in the trunk (try that in the GT) and have two cup holders inside. RE; 458, I ran Hennessy's Houston half-mile last year with an Italia present, so we had a back-to-back direct comparison. Without using launch control I was hitting 160 in the half-mile, the 458 never went faster than 148. At Circuit of the Americas just this past weekend I had two GT3RS porker drivers come up after a session and ask what I'd done to my car as, although they could stay with me on the bends, I was walking away from them on the long back straight. My car is bone-stock on the stock (crappy) tires. The 12c was 7seconds a lap faster than the GT.
The main reason I think that people choose the 458 over the 12c is emotional - its a much more visceral car (as noted above), makes lots of noise, has loads of buttons on the wheel etc etc. But, if you want a driving tool to go faster than anything else in its class, you will buy a 12c. Regarding price, the current over-stock situation in the US is all the more reason to buy one as its currently got to be the supercar bargain of the century! AS far as residuals go, they'll all end up similar in a few years whether F-car, Lambo, or Mac.