Interesting enough I'm just voiced my opinion on this very subject. Certainly at heavy Holyfield had much more of an impact. That said, he was the naturally bigger guy so it's not comely unexpected. Anyway, Holyfield get's the nod as far as I'm concerned. His fights against Bowe being one of the deciding factors.
Holy has the stronger resume overall, the greater longevity, and is more proven against bigger men. Spinks' really only has his dominant Light Heavyweight reign in his favor, and while it's highly impressive, it isn't enough to clinch it for me. I've got Holy just a few spots higher on my All Time list. Both probably crack the top 35 for me.
I pick Holyfield. Spinks was great but when he lost he quit boxing. Evander Holyfield went the distance.