One big factor affecting the feel in the weight of the keyboard behind the keys. An older Model M is going to have heftier components all around, and the keys will feel different than a newer model produced in the mid 90's or a Unicomp model, for example. In general, I find that the heavier the Model M, the lighter the keys will feel because those keys are sitting on top heftier components, which gives you a more solid base to type on.
And with SSK's, there is a lot less weight in the keyboard, so I would not expect it to feel the same as a full-size Model M.
Also, as mentioned before, the condition of the rivets will have an effect on the feel of the keyboard. Loose rivets will make it harder to actuate some keys. Send that keyboard to an experienced bolt modder, or learn to do it yourself to preserve that Model M for the next generation.