Well, the general consensus on here was that the only difference between Mk1 and 2 was mostly cosmetic.
But there are other differences.
Discussion on Flickr
Mark I :
Focus Details: AF (DC motor), with manual focus option
Magnification Ratio : 0.34x / 1:2.9
Image Stabilization; EF-S lens mount, exclusively for bodies with APS-C sensors. A lens hood (ET-60) and lens case (LP1019) are available as optional accessories.
Mark II :
Focus Details: AF (Micro Motor), with manual focus option
Magnification Ratio: 0.31x / 1:3.2
Image Stabilization with up to 4 f-stops of correction; EF-S lens mount, exclusively for bodies with APS-C sensors. A lens hood (ET-60) and lens case (LP1019) are available as optional accessories.
Going from this, it has a different focus motor resulting in the internal lens setup being somewhat different and possibly a newer IS system.
The focus fact is most interesting as this might either make it faster which is great, or more silent which is good for people that hate noises and don't care about performance.
However, a little further reading comes up with-
"The Mk II version of the original EF-S 55-250mm zoom lens still uses the DC Micro Motor (both DC and MM refer to the same motor technology). If it had the micro motor USM autofocus system, it would have the "USM" moniker and cost much more than its predecessor.
No change in the Mk II except for slightly different MTF chart curves and exterior aesthetics."
So you may as well buy the Mk1.