I would go for a Giant over a Ridgeback and don't worry about the sizes, Giant's come up small (oh the irony), eg. I am not exactly tall at 5'9" but my Giant MTB is a 20". I would be more like an 18" in virtually all other brands of MTB.
Good Luck
Hi, sorry to butt in, but I'm looking at similar bikes myself at the moment!
Any reason in particular you'd go for a Giant over a Ridgeback? Do they have a better reputation? It seemed to me that the Ridgeback (Velocity) gives you a better groupset for the money, so I was edging that way.
Also, HLaB, I know the Sirrus is lighter, but is it really *that* much lighter? I wouldn't have thought it'd make much difference on a short-ish commute, but I'm willing to admit I'm still learning
Thanks, and good luck eemike.