Personally I see very little difference bewteen true 720p and true 1080i and only few times have I seen what I thought was a difference. Admittedly my experience is limited, being only to our 32" CRT and on the two true HD projectors I've seen.
Indeed the Sony has the better res but res is only part of the picture quality story. Without knowing much about the Sony Qualia (only 'net reviews etc) I'll still venture to suggest that Digi Projection's expertise in DLP and theatre lenses would have the Mercury design still with the edge in colour stabilty & response and lens projection, both of which factor in majorly when creating a superb digital motion image.
But who knows (I don't!) an overall comparison between the two might come down to their scalers instead. Either way, I'd would be very happy to be lucky enough to have either one of them