Don't have a SN25P, but would imagine there's not much weight difference between the models. It is heavy once you put HD, DVD, RAM with heat-spreaders, video card with chunky heatsink etc. into it. I got the "camera bag" PF40 and "backpack" PF30 (picture on the Shuttle website is a bit squashed) bags. The backpack is very usable and well made, the stitching came apart on the camera bag... The backpack is also a LOT more comfortable to carry... Also, I don't think you can fit a keyboard in the "camera" bags.
HTH