I ride a Fuji Absolute for my commute...never tried the Specialized Sirrus, but they look like the same bike, pretty much. I'd see what you can get the better deal on. Many road bikes do not have attach points for discs...what's the point? They're heavier, and on a road you are unlikely to thrash your rims, or be slogging through mud puddles.
If you do get discs, who says you need them on both tires? Just put discs on the front...which is your most important brake for stopping anyway, thus spake Sheldon Brown. Then you can mount a conventional rear rack on the back of the bike.
There are racks made to work around discs (never tried 'em...I "mixed up" the brakes on my MTB, and my Absolute has standard rim brakes.) I've been told they don't hold as much weight, and tend to be less stable.