As far as M-4 medium vs T-34, pros and cons for both.
T-34 has better shaped armor - getting better protection from thinner plate, T-34 has wider tracks and somewhat lower ground pressure for poor traction conditions. T-34 has a diesel engine (unless I don't recall correctly) so somewhat less fire hazard potential. Russian armor plate was fair to very good quality depending upon year produced (later was better as I recall).
M-4 Medium - better mechanical finish, more mechanically reliable than any of its contemporaries, so more will complete the march to the combat zone.
75mm gun M3 on M4 Medium has a better AP shell (much more consistent in quality control for WWII - and by all published accounts - better AP performance), and greater ammo load.
M4 Medium main armament will have gyro-stabilization - not perfected, but useful.
M4 Medium has a functional radio, M4 Medium has a higher silhouette (by a small percentage) - but also a greater M/A depression for a better hull-down position.
Speed is about equal - the 21 mph quoted above was for sustained road march, sprint speed was about 30mph.
Crew efficiency - M-4 Medium has a 5 man crew compared to a T-34's 4 man crew, so there is a better distribution of duties in and crew efficiency in an M4.
Armor quality on an M4 - early models had notable numbers of tanks with sub-standard plate for cast, and M4 welded hull models - prior to late 1943, had a poorly designed glacis plate with too many weld joints. But all M4's produced from late 1943 onwards had good quality armor for thickness.
T-34/76 vs Tiger I, Tiger I advantage in a 1 on 1 engagement. M4(75) vs Tiger I - same.
T-34/76 vs Panther, depends on range, but greater room to maneuver favors the T-34/76. M4(75) vs Panther - same.
T-34/85 vs Tiger I, depends on range, but greater room to maneuver favors the T-34/85. M4(76) vs Tiger I - same.
T-34/85 vs Panther, depends on range, but greater room to maneuver favors the T-34/85. M4(76) vs Panther - same.
Numbers produced - significantly more M4 Mediums ...than any other WWII tank.
When T-35/85 met M4 Medium(76) in Korea & Middle East- M4 got the better, but that's attributable to training.
Overall - it's anyone's call...kind of depends on who is driving them, how many you have, and just how good your supporting arms are.
The Panther design was a copy to some degree of the T-34 but with poor manufacturing throughput. The key advantage for the Sherman was numbers. Other than that, in virtually all Tank versus Tank engagements on the western front, the German tank was superior in gun and armor but less so in manoeuvrability. Many American tankers were stunned and dismayed at the pronounced disadvantages they faced against the Germans.
The dismay was real, which explains the anger expressed in Belton Cooper's book on the "survival of an armored division in Europe." The American troops had been told repeatedly in training that their equipment was the best in the world, and essentially not to worry about the Jerries or anyone else. It did therefore come as a shock to find that this was not the case. However at least at the time of the M4's debut, it was one of the best mediums fielded in general stats. It had decent protection and firepower and decent mobility (on solid terrain). Steven Zaloga in his recent work "Armored Thunderbolt" (The U.S. Army Sherman in WWII), went so far as to suggest that in late 1942 the M4 was the best medium deployed in the West during the Alamein fighting. There were kinks of course. Early M4's suffered from armor flaws and combustibility and the high silhouette could be disadvantageous. Overall though....it was an excellent tank.....for 1942.
Problem was, as Zaloga well documents (I highly recommend the book as it goes into great detail on the behind the scenes polices for US Army procurement and weapons development), was that the parties in charge of R&D, rested on their laurels and didn't put enough stock into how future developments on the battlefield might impact things. There were more than enough warning signs as in late 1942 the first Mark IV specials were deploying as well as 75mm AT guns, both more than capable of defeating the M4's armor. Perceptions of "Battle need" were distorted and max production won out with only incremental improvements being forwarded. By the time of Normandy, the battlefield had changed significantly and the primary adversary of the M4 was that same 75mm AT gun vs. the older 37 and 50mm weapons. On top of that you had the same gun equipping the older Mark IV and the Panzerfaust. This led to excessive casualties. The M4 still "got the job done" but this misses the point that Zaloga and Cooper make, that lives could have been saved had more attention been paid to developing the M4's successor.
Ultimately Zaloga concluded that the M4 had been a successful if increasingly obsolescent weapons system that as part of a greater Industrial-military machine....gained victory in Europe. In all the stat counting and comparisons though it's easy to lose sight of the human cost though.