I bought a Chrome messenger bag (citizen) and a Tekstyl Omni sleeve that I use to get my laptop to and from school (2.5-3.5 miles depending on route).
I picked the messenger bag because I did not like the way my backpack was sitting while I was on the bike, but I was not happy with my rack + baskets for school, it was too fiddly. For a guy of average size, I would suggest going one size up to the Metropolis, I only got the citizen because I am female and quite tiny and felt the larger bags would dwarf me. I find that the messenger bag doesn't rob me of power like the backpack did and doesn't make my shoulders and neck sore either.
I picked the omni sleeve because it has some crush protection in it and was reasonably priced. I sometimes have to carry books in the bag with my laptop and I did not want to have to worry about the screen.
The sleeve fits in the bag just fine, but I don't think I could carry a bigger laptop very well in that bag. Mine is a Sony SZ140 which has a 13.3" screen and weighs 4 pounds. It is not uncomfortable by itself or with clothes in the bag, but it does suck to carry it + textbooks.
I like just being able to jump off the bike, lock it up and go when I get to school, rather than having to fiddle with panniers, and I needed something easy to carry around school and that could carry a pretty good bit of stuff. The messenger bag is working well for me, but your needs may be different.
I would not feel comfortable strapping it onto my rack because of the increased chance of injury to the laptop from the vibration and the chance of it falling off the rack (unlikely, but I'm not all that lucky). Plus the messenger bag provides protection from the rain here in Florida.