It's widely known that Model Ms and their membrane buckling springs were conceived as an overall cost-saving compared to the Model Fs and their capacitive buckling springs. But, does this matter? Are Model Ms without merit? This page will attempt to summarise the core differences between these two juggernauts of keyboard families to help you decide with one interests you the most. Note: due to the wide variance within both keyboard families, certain members of the Model F and M families are in mind during this comparison; the Model F/XT, Model F/AT, Model M Enhanced Keyboard, and Model M SSK.
Model Fs use capacitive buckling springs switches, which were the first implementation of buckling springs brought to market and feature a capacitive PCB underneath the buckling springs as the sensing mechanism. The capacitive nature of the design has two chief benefits over the later Model M's development; inherent N-key rollover and a much higher expected lifetime. Model Ms use membrane buckling springs switches, which are the second implementation of buckling springs that swap out the PCB of the previous design for a membrane and has smaller hammers and different springs that alter the key feel.
Capacitive buckling springs feel slightly lighter than membrane buckling springs, which is a pro or con depending on your taste. Most people tend to prefer the lighter feel, though. What some people don't universally prefer is the higher-pitched auditory feedback, with membrane buckling springs typically sounding more "baritone" by comparison. Many people report membrane buckling springs not feeling as refined as capacitive buckling springs in comparison, too.
Model Fs are typically made with hard (but also brittle) plastics, or metal. This makes many Model Fs feel more robust and strong when handled. However, Model Ms are usually made of more flexible plastic; whilst doesn't feel as taut as Model F plastic, it can absorb more damage. This is however offset with the Model M's Achilles' heel - the possibility of plastic rivet loss. In real terms, the case of a Model M is more resilient but the internal assembly is not. Model Fs technically also, have an Achilles' heel due to assembly foam degradation, although physical handing of the keyboard from the outside usually doesn't directly affect the foam.
The other possible weakness of most Model F's construction is the paint. It's possible to chip and scrape away the paint and its texture on Model Fs. By contrast, Model Ms have dyed plastic so there's no paint to chip away. It's possible to erode the texture of a Model M case with some abrasives, but it's still less susceptible to such an 'attack' than Model Fs.
Model Fs are technically closer to keyboard genesis than Model Ms, which means the first and the most common of the Model Fs, the F/XT, missed out on quite a lot of usability improvements made between 1981 and the launch of the Model M. These included the solidification of the navigation layout by the DEC LK201 keyboard, the near-finished solidification of the ISO layout by various terminal Model Fs, and lock-lights introduced by the Model F/AT. The most common of the Model Ms, the Enhanced Keyboards, have these as usability pros as standard. In fact, Model Ms solidified the ANSI and ISO layouts most use today, so most people trying a vintage Model M will still instantly be familiar with its layout. To get these features on Model Fs, you'll need to purchase the rarer variants that could potentially cost several times the price of even an '80s Model M. The Model F/AT specifically can have its alphanumeric section modified to match an Enhanced ANSI or ISO layout, but this requires additional cost and time.
Another score in the Model Ms favour is protocol. Out of all the Model Fs, the rather expensive Model F/AT is the only one that has a protocol and connector that is easily and cheaply converted for a modern PC (although of note, the also expensive Model F Labs reproductions are USB by default). The Model F/XT and all of the terminal variants require an active converter that is not stocked by major computer retail outlets or websites, requiring you to assemble your own or buy an expensive commercial option from orihalcon via eBay or tinkerboy.xyz. In fact, until the advent of Soarer's Converters, most Model Fs were totally unusable on modern PCs. Most Model Ms are either AT or USB HID compatible, allowing them to be easily used with modern PCs with at most a cheap active converter from Amazon for PS/2 Model Ms. And if you don't want to deal with any converter hassle with your AT or PS/2 Model M, Unicomp runs a service where they'll USB-convert your vintage Model M for you.
In summary, you could say Model Fs are technically better but Model Ms are practically better. Most agree Model Fs feel better to use due to the properties of the switches and the sheer build quality of the keyboard itself. However, most also agree Model Ms are easier to live with. Especially if you're not familiar with keyboards from before the modern period, the most accessible Model Ms field layouts instantly recognisable without much need for self-adjustment. And should something go wrong with your keyboard, fixes for Model M-related problems are arguably more documented due to the popularity of the keyboard, and Unicomp exists to cover you with a repair service and most parts if something goes really wrong. Whilst they're familiar with Model Fs, Unicomp doesn't have much or any parts spare to help people with their Model F fixing or restoration projects.
Regardless of your choice though, if clicky, tactile and above-average heavy switches are your thing, you'll likely enjoy either keyboard. Hopefully, this page has summarised well the most significant differences and pros and cons of both keyboards. If you need any additional advice or clarification, feel free to get in contact with me or make a post on r/ModelM!