personally, i carry my 700m in my backpack. it has a second thinner section that i can keep it separate from my books and stuff. i have a small face towel bunched up at the bottom of the pocket to prevent too much bumping when i put the bag down. i also make sure to put it in fan up so any random debris that's at the bottom doesn't get into the system. i also carry around the power cord and an external mouse in another pocket of my backpack. just for reference, i have an LL Bean backpack
for the xps, i dont usually bring that around, and if i do, i have a towel that i wrap around it to prevent too much scratching.
i find that just using my backpack is more convenient since i can use it for normal school use and also have my laptop with me. one problem is that it doesn't have the best interior, so the notebook does have a few minor scratch marks (actually, looking at it, i cant see many), and the corners of the white trim (inspiron series) are wearing away. that's what will happen more likely if you dont get a specific bag to carry it.
i think if you're going to be carrying books around with your laptop more often than not, just use a backpack (if it has a second pocket so it can have its own section). it seems easier than having to carry around 2 bags.