[von Poop]

Good question,
From memory the muzzle velocity and weight of shell of the 17pdr was about a third more than the Soviet 76.2, high muzzle velocity (though not too high) leads to greater accuracy and harder hit, also in the last years of the war there was limited availability of discarding sabot ammo (APDS), looking at this picture it was a hefty shell firing an advanced projectile:

British Tank rounds left to right: 2pdr, 6pdr APDS, 75mm (Sherman), 17pdr APDS, 77mm (Comet), 3" and 95mm CS.

from:http://www.flamesofwar.com/Article.asp?ArticleID=324, I reccomend a look there and follow the link to 'Guns versus armour' at the bottom.
Good luck,
Adam.