I was recently reading about various second generation tanks and I noticed that there seem to have been two different ideologies. The M60 and the the Chieftain both have extensive armor and have hundreds of millimeters of armor from the front. They were very heavy and relied upon their armor to survive. The AMX-30 and the Leopard 1 on the other hand seem to follow a different idea of being light and mobile to avoid being hit. They both are much lighter than the other two and have much weaker armor, from what I could find less than 100 millimeters frontally. So of these two ideologies, which was more effective? Did the technology of the day mean that tanks would be easier to destroy than to protect or was it worth it to be heavier and slower but have more armor? I know there wasn’t a full conflict to really show which worked better but I was hoping for information on the effectiveness of the opposing systems.
There's a few ways to look, and take this:
Of the four tanks you mention, all but the Chieftain have had extensive service lives. Broadly speaking it would be possible to call them all "successful" in the regards they have retained military utility for decades (the Chieftain still served in UK forces for some time, the Iranians didn't seem to hate it, it just wasn't quite the success the M60/Leo 1 and to a lesser extent, the AMX 30 was).
All of these designs however, arguably had their various "approaches" become obsolete with the revolution in composite armor/ERA in the 1970's-1980's that made armoring against KE and CE type rounds practical (after that point, you basically have some balance between ERA or heavy composite armor, the number of MBTs seriously designed to have "light" armor dries up, while the heavy conventional armor of the other designs simply couldn't compare to conventional arrays plus ERA, or more advanced composites). In this context it's arguable both approaches were equally surpassed by a revolution in technology vs one strictly being "the future."
Which was best in "combat" is rather difficult to pin down. Of the four tanks you mention, the M60 by far and away has the most extensive combat exposure (US use in 1991, extensive Israeli employment from 60's on, Iranian use in Iran-Iraq war, ongoing Turkish employment, etc etc) and while there's questions to if it would have been all the tank to beat the Soviets, it's certainly done rather well for a tank that dates back to 1961. AMX 30s have done okay although their use has been a bit less extensive (French in 1991 against fairly modest opposition, various Arab states), the Chieftain also saw some pretty extensive combat in Iranian hands, while the Leo 1 has seen very little.
But going off that discussion, it's hard to point at one design or the other and say one type or the other performed lightyears better than the other would have given similar crews and employment (Israeli Chieftains being a fun "what-if" given how close that was to happening).
As someone who has training and some operations background in armored operations:
Speed in tanks is useful in as far as limiting exposure from moving from hard cover to hard cover, but it's less useful when dealing with making contact with the enemy (as you are fully exposed). Additionally moving too aggressively often exposes tanks to danger (the faster you go, the less aware you are, the more likely you are to be shot). In practice, we tended to operate at "creeping death" which might have been 10-15 MPH as it kept us from going full bore into trouble, and made formations/observation more effective (command and control is easier, you have more time to observe/"scan" terrain before you are in danger etc).
As a rule, M60, M1A2 SEP v2, T-90, Tiger II, Sherman etc, etc, etc you do not want to get shot in a tank. The armor makes it less likely you will be damaged/killed, but there's still the chance your enemy gets "lucky." More armor is nice, but it isn't a license to lose your caution. However, if you are in combat more armor so long as it does not compromise the rest of your tank's capabilities (i.e., you can still go at a reasonable pace, don't get stuck, won't catch your transmission on fire) seems to be useful most of the time.
From these comments:
The ultimate goal is a tank that has max protection, max mobility. From a professional perspective though, exceptional mobility pays off less than armor in practice (you ALWAYS want to be protected as much as possible, while sometimes, you want to to fast).
In regards to HEAT:
Basically within a narrow strip of history, there was no reasonable armor to HEAT rounds which went from being a "close range, not common" weapon to a "we've put HEAT warheads on everything" tier threat. More advanced composites and ERA undid much of this, but again in the 60's there was a lot of question to if armor had just had it's day, and it should be pretty indicative of the two tanks mentioned in the question, the two that are still in service are usually pretty buried under ERA if they're still in use.
Adding onto this, could someone explain why the major nations thought that HEAT made tank armour obsolete?