I've gotten equally bruised up on SM and BTMR, but I'd agree that BTMR is the "rougher" of the two. By rougher I mean that there's more bouncing, jouncing, jostling, and vibration. SM by comparison is faster and harder with steeper drops and more forward momentum so that it hits you harder if your elbow gets slammed against the inside of your seat, but it's a smoother ride in terms of bumps and thumps.
One thing to note is that while BTMR's ride vehicle can seat two people, SM's is more compact, and that's a great help if you're riding with people in your party who are significantly larger or smaller than others. For instance, if a child and an adult ride the same vehicle on BTMR, inevitably what ends up happening is that whenever there's a sharp curve, both people go sliding across the bench seat and whoever is on the inside of the curve gets squished. If one person is a 65 lb child and the other's a 185 lb adult, that can be a problem. There's no real way to avoid it since the track has curves in both directions so no matter which side you're sitting on, at least one time during the ride you're going to end up pressed against the inside of the vehicle with the other person clinging to the bar trying not to slam into you as their body slides across the bench and collides with yours. So if it's your first Disney trip and you've never ridden BTMR before, it's important for the larger person to be aware of this and make sure to cling to the safety bar for dear life to avoid flattening your kid! And if you're alone in the ride vehicle, then YOU have to make sure to grab tight to the bar and hold yourself stable through the banking curves, otherwise you'll slide across the bench seat and slam into the side of the vehicle, which I can tell you is a lot more painful than anything Space Mountain can do to you because the SM vehicles are divided into three separate compact seats that don't allow for much sliding around.