Many people have little idea of how these tanks, the main tanks of their respective nations during the war, compared to each other. Alot of people know about how weak the Sherman is supposed to be, and how powerful the T-34 was though to be, but these things are not correct. All these tanks were about the same.
There were multiple models of these tanks. They can be broken up into 2-3 groups depending on the tank.
The early T-34s were equipped with 76.2mm guns and had four crew. They entered service in late 1940. Models with a redesigned turret housing an 85mm gun came into service starting in 1944 (these had five crew). Throughout the years, turret armor was improved in various sub-models. Much of the reason for the T-34s great reputation is that when it first started fighting, it was very difficult to kill due to it's good armor protection.
The early M4 Shermans came around in mid 1942 with a 75mm gun. Beginning in 1944 (when the U.S. realized it was out-gunned), Shermans with 76.2mm guns began to enter service. Also upgrades to off-road ability and armor were made to Shermans with either type of gun. The reason they have a bad reputation is that most of their fighting was in 1944, much of which was against Panthers and Tigers.
The German MkIV was produced as early as 1937. However, until the MkIVF2 entered service in 1942, these tanks were equipped with short-barreled 75mm guns, not to useful for AT work. The F2 had a medium-legnth barrel and was much better. Later models would get armor improvements and a slightly better gun.
How did these tanks compare to each other? One must select contemperary models.
In 1943 the Panzer IVG, M4A3 Sherman, and T-34 model 1942 squared off against each other.
Speed: The T-43 takes the prize here with 55km/h speed (~40 off road). The Sherman could only make a 42km/h road speed (I am not sure about off-road, I have seen estimates between 25-31 though), and the MkIV a 40km/h road speed (~16 off road). The reason for this was track size and suspension. The MkIVs had very thin tracks, so could not get very good off-road performance.
Firepower: The MkIV had the best gun (higher velocity) by far and better optics too. The Sherman was not far behind in optics, but did not have a very good gun. The T-34 had a horrible gun for its size and poor optics as well. Each gun could penetrate the other tanks at normal combat ranges, except that the T-34s 76.2mm gun could not penetrate the hull of the MkIV except at close range (nor could it penetrate the Sherman front except at close range...).
Protection (frontal): The T-34 and Sherman are about the same. The T-34 had equivalent thickness (taking slope into account - NOTE: besides for thickness increase, the slope provides addition protection against most ammo) of ~80mm (varied due to curve) for the turret and 90mm for the hull. The hull was also very sloped. The Sherman had ~88mm in the turret and 91mm in the hull (slightly less sloped then the T-34). The MkIV only had 51mm in the turret and 82mm in the hull, all with very tiny slopes.
Special: The early Shermans were prone to catch fire. The T-34 only had a two-man turret, so the commander had to be the gunner too, reducing efficientcy. Also, the T-34s were often hampered by lack of radios (this may have been largely corrected by 1943, I'm not sure). On the other hand, the Sherman and T-34 were more reliable (mechanically) then the MkIV.
The later varients of the Sherman and T-34 in late 1944 and 1945 were greatly improved. The MkIV was not, but then the Panther began to appear in signifigant numbers, so 1945 comparisons would be T-34/85 Model 1944, MkVG Panther, and M4A3(76)W Sherman.
I have used multiple sources (including Combat Mission, a well-researched WWII tactical Combat game) and the online site
http://www.onwar.com/tanks/index.htm
Note that if you go to that site, the slopes are given the opposite of what they are usually given as, so 90 degrees is actually zero degree (no sloping) etc.