He might have feigned ignorance were they discussing any other subject, made flippant comments about small differences and big differences, coifs to mohawks and bananas to oranges, the sort of things that would usually illicit exasperated laughter from the blond companion were it anything else. Instead, he remained very quiet for a time, watching the moisture build and threaten to fall from her eyes. This wasn't just it isn't the same or he's not you, as much as the arrogant and selfish (envious?) side of him might like to think so. He didn't want to make assumptions, knew he would never intentionally hurt Rose (except for that one time) regardless the incarnation, which is why this made so little sense, but he couldn't ignore the hurt and unmistakable fear first in her eyes and then in the waver of her voice.
The Doctors' eyes hardened, a grim expression clouding his face when he leaned in and canted his chin down enough to look Rose square in the eyes. “How different?” he asked, voice wary and mimicking the look in his eyes.