Howdy
Just as the '73 was an improved model over the '66 in the 19th Century, the modern '73 is improved over the modern '66. a quick look at the history will help understand the differences in the modern guns.
The '66 was concieved as an improvement over the original 1860 Model Henry. The '66 was the first lever gun to incorporate a side loading gate, eliminating the open slot at the underside of the Henry magazine. The slot was there to allow the finger tab for the follower to slide the length of the magazine. Not only did this improvement mean the '66 could be loaded more conveniently it also meant a wooden forestock could be applied to the barrel, to keep the hand from getting burnt as the barrel heated up. The original '66 was chambered for the same relatively underpowered 44 rimfire cartridge as the Henry, and used the same gunmetal alloy for the frame that the Henry did. Gunmetal was the 19th Century term for a bronze alloy often used in firearm manufacture. Because the 44 rimfire cartridge was relatively weak, the yellow gunmetal frame sufficed for boty the Henry and the '66.
The 1873 Winchester was conceived as still a further refinement of the lever gun. The '73 was introduced with the new centerfire 44WCF cartridge that we usually call the 44-40 today. The 44-40 carried almost half again as much gunpowder as the 44 Henry rimfire cartridge, so to be strong enough for the new more powerful cartridge the '73 was not built with a gunmetal frame. The first '73s were built with iron frames to take the higher powered cartridge. Later the frames were built from steel. Another improvement on the '73 was the lever operated disconnect, which disables the trigger until the lever is fully closed. This was included in the '73 design because of the need to prevent out of battery discharges with the more powerful ammo. It is not a modern 'lawyer dictacted' device, it existed on the 19th century '73s. About the only other significant addition to the '73 was the sliding dust cover on top. All the toggle link guns were completely open on top, the bolt is clearly visible from the top. The dustcover on the '73 slides back for ejection when the lever is first worked and stays back. It can be manually slid forward to cover the top of the action whenever the shooter desires.
The 44 rimfire cartridge is no longer available today, and both the Henry and the '66 are chambered for modern centerfire cartridges. The modern brass frames are perfectly capable of withstanding the recoil thrust generated by modern SAMMI spec Smokeless ammo. However repeated use of high powered ammo can result in frame battering that can ruin the headspacing. All the improvements of the original '73 have been carried over to its modern counterpart. The frame is made of steel, the dustcover and the trigger safety interlock device are also present. Even though the frame of the modern '73 is made of steel, the toggle link design is inherently weak, and modern '73s should not be subjected to ammo more powerful than SAAMI spec. One thing most shooters do not like about the modern '73 is the spring on the trigger interlock device is stiffer than it needs to be and it requires more force than it should to squeeze the lever shut in order to operate the trigger. The interlock device should not be removed, there are after market springs on the market that can be substituted to make the interlock less noticeable.
For takedown, the '73 is the easiest to deal with. Removing one screw allows the sideplates to come of and the entire action is acessable for cleaning. The '66 requires the lever pivot screw to be removed to remove the sideplates.
Personally I am prejudiced toward the '73, I've never been much of a fan of yellow guns. I prefer the easier takedown of the '73, and I think the trigger interlock is an important feature. Plus, there was never a movie made called "Winchester '66".