I test drove both last week. Could have gotten the Sorento for a few hundred less then the Sportage but I went with the Sportage.
Sorento is a bit wider, a bit longer, and definitely has more cargo room in the back. Thought the interior trim looked a little more refined than the Sportage.
But the Sorento seemed a bit heavy for the powertrain package. It felt sluggish. There was a definitive difference with the Sportage - much more nimble in both manuevering and throttle response.
I went from a Suburban (12 years) to a Spectra (18 months) so I've been at both ends of the stick. The Sportage just seemed to offer a bit better combination of price, size, performance, and value.