Why Is Two Clubs the Preferred "Strong Two" In Bridge?
2 Clubs is the most useful because it leaves the 2 level open for you to describe your hand.  Your side has the preponderance of strength, and thus your side would benefit the most from the extra bidding space.  Conversely, keeping 2 Spades preemptive prevents your opponents from starting to investigate their hands on the 2 level.  If your opponents have a heart fit, they can start to investigate it with a 2 heart bid over a preemptive 2 clubs, while they would have to start with 3 hearts over a preemptive 2 spades.

The system of responses you describe is one that exists in various formats, called Step Responses, where the weaker partner quantitatively reveals the strength of his hand to the 2 club bidder so that he may make a guess as to whether end up in game or slam.