The best thing about the Sirrus is that it doesn't have a suspension fork. As hybrids go, I think it's one of the better designs for that very reason; it's closer to being a "flat-bar road bike" than other current hybrids, most of which seem to just be comfort bikes with slightly larger wheels.
A MTB with slicks is a perfectly good compromise if you think you may want to do more off-roading in the future, but the truth is that a $450 MTB probably won't stand up to much of a beating. Whereas a Sirrus, used on pavement, paths, and gravel, should last indefinitely.
Road bike prices are coming down: there's a whole class of entry-level road bikes in the under-$500 range now. But if that's not what you want, the flat-bar road bike -- or "fitness bike" as they're starting to be called -- is a reasonable compromise. Personally, I'd stay away from low-end MTB's, even with slicks.
Training: 2002 Fuji Roubaix Pro (105 triple)
Commuting/Daytripping: 2001 Airborne Carpe Diem (Ultegra/XTR, touring wheels)
Commuting/Touring: 2000 Novara Randonee (Sora/Tiagra/LX, fenders, lights)