I commute by both road and tourer. Once or twice a week I'd use the tourer to take in a few days-worth of clothes so i could use the road bike the rest of the time. Much depends on how much gear you have. Rather than a rack, a Carradice saddle bag might be a perfect answer for you, and would enable you to use the road bike every day. personally I wouldn't recommend a backpack - sweaty and alters your centre of gravity.

And everyone else is right about the saddles, if you get one that suits you a road saddle is comfortable.