Spyder will give a strong magenta cast after a few years to tell u its time for a change. i1 will maintain the color accuracy, but I think the light metering will be slightly off. That's based on my memory...
Anyway I have both Spyder (though its just a Spyder3Pro) and i1display pro in my hands, the i1 is way more accurate compared to the prints. I calibrated with displaycal, which gave even more accuracy but a lot more effort.