at the risk of re-igniting this and sorry for a long post...
I do not claim any military experience and my background is medical rather than anything else (although we are trained to assess data and evidence and I have a significant interest in history and military history).
My understanding is that the advantages the Sherman had over the Panther and Tiger were:
- crew survival in the event of being taken out: my understanding is that overall/in North West Europe something like 80% of tank crews could bail out of a knocked out tank to crew another tank; a T-35 had something like a 20% survival rate; the data on knocked out tanks came from the doctrine across all sides in WW2 of shooting a tank until it is definitely dead and that generally means on fire whether or not the crew have bailed out; I don't know how survivable the German tanks were but one combat records that of 3 Tigers destroyed by IS-2 tanks only two crew members survived.
- logistics: these tanks could be mass produced, easily shipped from the USA to the combat theatres and by standardising on essentially a single design spare parts were easier to supply (as opposed to a German unit that might have some Panzer IVs, Panzer II command tanks, Panthers, Tigers and some StUGs).
- reliability: this means that when you send a battalion to an attack about a battalion's worth of tanks arrive at the start line, for the Germans perhaps half the battalion of Panthers would make it without breaking down (and they could only move at night and often lacked fuel).
I think that all three of the above points work with the adage that amateurs study tactics but professionals study logistics as all three points are logistical benefits to the Sherman over the Panther and the Tiger.
The description I have read that is best is that the Sherman was good enough to win with a gun that was good enough for some anti-tank work and firing HE rounds at the time it was developed (look at the poor M-3 Grant/Lee with two main guns...); with enough Shermans you cold throw enough at a small German unit to get the flank hits that would knock out the tanks and the losses of trained personnel were acceptable even if you lost the tanks back to repair or salvage. A description of an action in North Africa is that a Tiger is merrily brewing up Sherman tanks so a (or at least one) 17 pounder anti-tank gun was brought into range and started firing at the Tiger, the Tiger then turned it's frontal armour to the 17 pounder meaning it was exposing flank armour to the Shermans and thus although not penetrated or knocked out it withdrew: this is a victory because the New Zealander force achieved their goal. The Germans also talked about the danger of the Sherman tank in "herds".
The other thing at least some Allied users of the Sherman had (I know the British and Canadian Armies had these but not sure about the Americans) was artillery forward observer tanks which allowed them to call in artillery strikes rapidly on the enemy positions.
I've recently been to the Tank Museum at Bovington and they have some Tigers and at least one Panther as well as Shermans and the size difference is significant meaning the German tanks will be easier to spot as they are taller and can't hide behind the same cover.
At the Tank Museum I bought a book called The Tank Commander Pocket Manual 1939-1945 that is mostly extracts from the various doctrines about tank use - for example the US Army 1942 Field Manual says medium tanks "are used to (1) lead an attack against an enemy whose position and strength are known (2) support by fire the attack of either light or medium tanks" while under mission and echelons of attack it says "the mission of tanks in the armored division is to attack and destroy vital hostile installations such as command posts, supply installations, reserves, and artillery" (I hope you all appreciate that in making a direct quote I misspelled armour for you and inserted an Oxford comma which I don't believe in). Later the same field manual talks about what ammunition to use against different targets and in all cases the armour piercing is for use against hostile tanks whether 37mm or 75mm.
Even a British Army pamphlet from 1939 says that for counter-attacks the infantry tanks of the army tank battalions are eminently suitable for use against enemy AFVs and they then go on to give detailed advice about tank-versus-tank combat.
Finally there is a quoted passage from the commander of the first unit of Tiger 1s in action in Russia - one tank's transmission died getting onto the train to Russia but it was taken on the train with them, then two more were out of action with transmission trouble on the first day in Russia... which left one tank to actually go out and patrol! When the Tigers did make it into action three of the four were damaged with one write-off but the crews commented on all being able to evacuate and feeling safe in them. So, a good weapon if it makes it to the fight.
