It's not the weight so much; it's the balance.
I hiked around in the mountains for a while yesterday with the a 642 in a Nemesis. It was no problem. More weight would have started to get uncomfortable, just because of the unbalanced weight.
When I load them down with stuff (GPS, water, etc.), my pants can be like a backpack. They're comfortable when properly balanced, but after a few miles hiking up and down hills and canyons, an unbalanced load causes premature fatigue, back pain, etc.
I wouldn't let that stop me from getting an SP101; I wouldn't, however, trade one for the other, myself.
Bottom line?
I think these are two different guns, that will serve two different purposes.