Western tank designs mostly prefer 4 crew members team, commander, gunner, driver, & loader. But Russian designs & all its derivatives prefer 3 crew members team, commander, gunner, driver, & an auto-loader. Can you guys tell me the advantages & disadvantages of both the design concepts? In other words, I want to know the pros & cons of a manual loader vs auto-loader. And since a 4 crew members team makes the tank bigger & heavier to accommodate 1 extra person, does it also offer some significant benefit to justify the bigger size, extra weight, & more cost?
Advantages of 4 man crew with manual loader:
1. Superior situational awareness.
2. More space inside and outside crew compartment for modifications.
3. Simple maintenance activities take much less time to complete.
4. Reloading of the tank's bustle storage takes little time with 4 crew members.
5. More close range protection is possible, both loader and commander can fire mounted machine guns.
6. No problematic autoloader problems, human loader doesn't need repairs.
7. Rate of fire is usually more than the autoloader.
Disadvantages of 4 man crew with human loader:
1. If the tank is running on rough terrain, the loader will find it difficult to load the main gun, reducing rate of fire.
2. More weight for same armour level.
3. Become large tanks, easier to hit thanks to width.
4. More crew members means more money spent on training and feeding them.
5. Since Loader is human, if he is hit, someone else will have to take his job in the battlefield, complications might occur.
Advantages of 3 man crew with Autoloader:
1. Lesser weight for same armour.
2. Autoloader gives stable, reliable rate of fire.
3. Smaller target.
4. Smaller engine and lesser fuel is required, increasing mobility.
Disadvantages of 3 man crew with autoloader:
1. If autoloader malfunctions, manual loading takes at least half a minute in any smoothbore weapon.
2. More repairs.
3. Lesser space for crew and modifications.
4. Maintenance is much tougher with lesser crew members.
Western designs put emphasis on situational awareness which is huge benefit and advantage over Soviet tank designs, western designers don't worry if weight exceeds 60 tons, they focus on protection. However in the future, Western tanks are also moving to autoloaders with 140 mm caliber and unmanned turrets to reduce weight significantly, but not two-piece ammunition but single piece ammunition which makes for more length than current 120 mm rounds, a significant advantage over Russian design "Armata", future Ukrainian designs and Chinese Type 99KM.
Current NPzK with 80's 90's technology ammunition could penetrate 1100 mm of RHA with German ammunition (old WHA alloys, much inferior to DU alloys those days) and 1050 mm with Swiss ammunition. Imagine, if Germans use newer Tungsten alloys with composite sabot technology and DM-63's propellant, what would happen!
Well, US already has several unmanned turret tanks designs, all with excess of 1200 mm equivalent composite armours in the frontal aspect, an extraordinary achievement while keeping weight below 55 tons.
The future is coming. It's just that we aren't ready for it.
) with 3 crew & an auto-loader, that will keep the weight below 50 ton but still be very advance & well-armoured. Having gained experience & knowledge on Arjun, this should not be very complicated for DRDO. 
Not to mention that comparing T-90S with Bhishma is a futile exercise, Bhishma was an archer and strategist without equal in the Mahabharata.
