If your over 6' or over 29er's make more sense than if you are <6'

For getting upto speed, yes a 29er should be faster when rolling, but it will always depend on the tire, a high rolling resistiancy / heavy 29er will be slower than a light weight 26", if you are commuting, and looking for a slickish tire, this shouldn't be an issue.

The rack fitting could be an issue, as the mounts on a lot of 29ers seem very low; including the Hardrock, you may have issues with the roundstays (the thin rods between the bike seat stays and the rack) being too short dependint on rack manufacture.