As an easy experiment, try putting the saddle from your MTB on your roadie, and see if that makes the difference... Also, I second the "fatter tires at lower pressures" opinion. Since you specify the saddle as the source of your discomfort, I can assume that your position, posture, and handlebar ergonomics are okay for you.
Although looking at that picture, I notice that the handlebars are lower on the roadie than on the MTB behind it. You may just want to try a stem extender (or adjustable stem, since I think that's a quill stem, isn't it?) on the roadie to increase the handlebar height, since it might just be the different position you are in on the saddle that is causing problems.
Last edited by David Bierbaum; 02-20-13 at 09:52 AM.