Outwardly the Systema roll looks different from the Aikido roll. That is only as far as the appearance goes. The principle governing
the two are essential the same though the form and shape between them may be different. Let us look at how the Systema and the Aikido practitioners do the forward rolls.
The Systema roll
The practitioner aligns his torso square with the direction that he is going to roll. He lowers his body vertically downwards to a squatting position and gather his two palms in front of his chest, similar to the movement of a swimmer at the start of the breast stroke. As he bends forward and downwards he sweeps his hand outwards in a big arch again like the shape of the end
The Aikido roll
The Aikido practitioner aligns his body sideways (hanmi stance) parallel to the direction of the forward roll. He bends his body forwards from the waist at the same time gradually bends his knees. He extends his right and left hands forward at the hip level, the right hand in front and the left hand trailing slightly behind the right hand. With the right hand pointing towards the torso and the left hand pointing forwards, both palms facing the floor the two hands act as an elastic bridge in the shape of a arch. As he projects his body forwards and downwards he maintains the bridge-hand form until the time when he is about to make contact with the floor. Just before the body slam the Aikidoka swings his right hand forwards and downwards to touch the floor ahead of the rest of the body. The right hand acts as feelers and sensors to guide the roll along the right shoulder down the right hand, cross the spine to the left side of his body. To soften the impact of the body hitting the floor he slams his left hand on the floor as he rolls out on his left side. Similar to the Systema practitioner, the path of the roll is diagonal starting with the right shoulder, down the right hand across the spine he comes out of the forward roll on the left side.
The shared rolling principle
In Systema rolling the path of the roll is diagonal starting from the shoulder on one side of the body down the same side hand, then across the spine on the torso and exit on the opposite side of the body. The Aikido practitioner also executes the forward roll along a diagonal pathway crossing from one side of the body to the opposite side.
What is different between the two are the forms. The Systema practitioner approaches the roll with his torso square with the direction of the roll. He initiates the forward roll by lowering his body vertically by bending his knees, then as his shoulder make contact with the floor he gradually eases his hips and legs over his head.
The Aikido practitioner likes to stand sideways parallel to the direction of the roll. He initiates the roll by lowering his knees slightly and bends his body forward and downwards at the waist. The as he goes air-born with his body he projects his entire body forward and downwards.
Essentially the Systema practitioner executes his roll by first "sitting down" and then dips his upper body forwards to exit from the roll. The Aikidoka's roll is more of a rotational movement in the vertical plane starting from the standup position without a interim movement such as "sitting down" of Systema.
My take on the two ways of rolling
Personally I prefer the Systema roll more than the Aikido roll. I find that the Systema roll tends to be softer and easier on the body as the interim vertical "sitting down" movement resolves about 80% of the impact before the body makes contact with the floor. Thus by the time you hit the floor you only need to take care of the last 20% impact force. On the other hand as the Aikido roll is essentially a rotation in the vertical plane without any interim movement you have to resolve 100% of the impact force by means of dynamic body rotation. Sometimes the overturning force is so great that body rotation is not enough to resolve the impact and that you have to slap the floor forcefully with your hands to dampen the impact of the body slam and also to check the body from spinning out of control.
Rolling on a hard surface
In my opinion if you practice in a dojo that has mats there is little difference in how you want to roll as the mats are an additional mechanism beside your own ukemi skills for damping the impact force. On the other hand if you practice on a hard concrete floor without the protection of mats the Systema style of rolling is kinder to the body than the typical Aikido roll. Likewise if you have to take a breakfall any where outside the safety and comfort zone of the dojo eg on concrete, timber, tarmac, earth, sand, rock, etc it is safer and less damaging to your health and body to roll in the Systema style.