You're not going to find any effective, measurable difference over the internet. Only in person. Remember, it's just a shorter Model M with a different controller. Because of it's shorter length, it may seem more tight or less flexy in some respects but honestly, there really isn't any significant difference between a Space Saving Model M and a full size Model M of the same era if they're in the same condition. I have full size Model M's that are every bit as solid, tight and special as space saving Model M's are.
The biggest difference is being able to move your mouse in closer and position the keyboard differently on your desk.