I have had a M-100 for years, my Dad actually bought it new in 1960. Like all Winchesters the pre 64 years were of better quality.
The obvoius external difference being the post 64's have stamped basket weave checkering the pre 64's have the traditional cut diamond checkering.
There are five Win. M-100's in our hunt camp alone, yes the odd time they jam/malfunction, but no more or less often than the Remington 742's and Browning Bars. We have a fella with a Remington 760 pump in camp that was ging to wrap it around a tree a couple years back because when Mr. Buck was standing there most patiently in front of him and he clicked all five rounds through the gun none of them going off. The problem was a weak fireing pin spring. A gun is a mechanical device, the auto's being the most finicky, any mechanical device is prone to malfunction. Any of you fellas that have heen in the military know that gun cleanliness is high of there training list, and for good reason.
In my experience with M-100's, and I can say it is very extensive in the bush experience.(39 yrs) Any auto jams for a reason and in 99% of the cases it's something being dirty. In the case of the M-100 keep it clean and lightly oiled except for the gas system parts which you keep clean and "DRY".
Very important to clean your clips and shells at the end or start of everyday. Get all the spruce needles, junks of bark and other bits of crud out of your clips. Then wipe all your shells with a lightly oily rag before loading them into the clip. The reason this is so important is that dry dirty shells will hang up and or not completely enter the chamber. In the case of the M-100 this will not allow the bolt head to turn and lock into position, preventing the gun from firing. For the same reason don't ever let the bolt down slowly, let it slam shut.
The old fella that taught me about the clean the clips and shells and let the bolt slam proceedure actually had a Browning Bar that used to jam habitually. Over the many years since I have been following his advice my M-100 has gave obsolutely zero issues.
So all that being said look out for the following;
+has the firing pin recall been done? (Safety issue)
+are the gas system components pitted and rusted? (Some guns have steel parts other model years stainless steel)
+is it a pre or post 64?
Since the M-100's are tricky to take apart and even trickier to put back together correctly, I would strongly and highly suggest you take this gun to a competent gunsmith for a thorough examination or if you have a friend who is very familiar with M-100's let him do it.
They were well made, very reliable and accurate guns. Winchester discontiued them as they were expensive to make and they couldn't compete with the cheaper to manufacture Remington M 742. Plus Remington was a marketing machine. The M-100's and 88's were very popular here in eastern Canada but not in the US where the main gun market is. The M-88 which is the lever version of the M-100 was discontiued for the same reasons.
They have a following and command prices depending upon condition and year of manufacture from $450 to $550.
To help answer part of your original question a Marlin 336 has a reputation of being a very reliable gun.( the older production models) Is it better than a M-100? Your trying to compare apples to oranges here. Depends on what you like and what your going to hunt with it. Me, I would have both!!!
If you live close to my location, PM me, and bring it on over and I will strip it down and take a look.