Ok, just for sh*ts I'm going to compare it to my favorite medium tank in the game, the T-64A, even though this is a combined arms game and comparing two units as if they were only going to face one another is a fallacy. You've already dismissed that since you assume everything must have an analogue. http://i.imgur.com/wZxTRne.jpg
Both are availability 0/16/12/8/0
I'm not going to bother calculating the effect of the "small" size because I don't know the value of the modifier yet.
That that the T-64A and STRV 103B have chances to hit % of...
...respectively at 2100m.
The STRV 103B has a 50% ROF advantage, let's factor that in as a chance to hit for every cycle of the T-64A...
That's a pretty nice boost in CTH per cycle relative to the T-64A.
They are largely comparable otherwise but the s-tank has the edge in a standoff engagement. Ya, I guess for 5 points more the S-tank does look pretty damn nice; but wait, what's this? It hasn't a turret nor can it fire on the move? It's also got poor mobility and range? I guess there is something one trades off for other qualities after all!
Maybe a smart player would use that mobility and ability to fire on the move to their advantage and use one element to suppress the S-tanks to remove their accuracy and ROF advantage while another element moves up to a close range? Maybe?