There's a very common confusion in solar terminology ,where "MPPT" and "PWM" are being compared.  This is sort of like comparing two different cars where one has fuel injection and the other uses spark plugs.  The reality is that BOTH use spark plugs, and fuel injection only really helps in certain situations.  (it's not an "either-or" comparison)  And there's also a big cost difference.

It's difficult to simply describe how PWM (Pulse Width Modulation) works in a controller without getting confusingly technical, but I'll try.  "Basic PWM" regularly interrupts the power (through a filter) to lower the average voltage to what the load can use, which can be very inefficient since it spends some (and maybe a lot) of its time "off" not sending power to the load.  It's fairly efficient when the panels are providing the right amount of voltage for the panel (and the PWM is almost always "on") but the power of any additional voltage is "lost".  So for example, a 28v panel would be VERY lossy when connected to a 12v battery because it'd be "off" for around half its cycle.  Their efficiency is directly related to how big the voltage difference is, so if your panels are closely matched to your battery, they can still be fairly efficient.  At any rate, this DOES prevent a higher voltage solar panel from over-charging your battery, so it is a legitimate way to control a solar panel.

The other method is to use PWM to control a "buck regulator", which is always transmitting power but is using two different current paths, so can be *very* efficient.  Unlike basic PWM, buck regulators "trade off" excessive voltage for more current, and thus can provide *more* current to the load than the panel is providing, making them extremely efficient at any panel voltage or battery charge state.  There are some great videos on YouTube that describe how buck regulators work, check them out.

*Both* methods use PWM, so saying a controller is "PWM" isn't helpful, because ANY real solar controller uses PWM.  If you can spot schottky diodes and a large inductor, it's probably using a buck regulator.  The inductor makes the controller bulkier and heavier, so if it's really small/thin and lightweight, it's probably just using inefficient "direct PWM" and not a buck regulator.

Now that we're done with that, MPPT is "Maximum Power Point Tracking".  This isn't a method of power conversion, it's a method of *control*, and is almost always done with a small micro-controller (tiny computer chip) on the board.  In theory it could be used to control a basic PWM conversion but I've never seen it - probably because it'd be a waste of money to put in a MPPT chip just to waste all that power with basic PWM.  MPPT constantly calculates power delivered to the load, and is constantly tweaking the voltage/current tradeoff in the buck regulator, trying to find the optimal conversion.  This might sound unnecessary if you can just cut the voltage in half to double the current it wouldn't change the power.  But solar panel output is *not* linear.  Doubling the current draw might drop voltage by 40%, 50%, 60%, etc, and the result power might be 80%, 100%, or 120% of what it was before.  Obviously 120% is better, so it moves there and then tries adjusting again to see if it can continue to improve.  This is the controller "tracking" the maximum power point, along the MPP curve.  The shape and position of this curve varies depending on the panel and the conditions (how sunny it is, angle of the sun, etc) so it's necessary to track it and adjust the conversion rate if you want to squeeze every watt out of the panels as conditions change. (and to have the controller work efficiently with more than one kind of panel)

So on the front end, the MPPT chip is adjusting the buck regulator to pull just the right amount of load to maximize solar panel power.  And on the back end the buck regulator is efficiently converting voltage down and current up so that close to 100% of that power is delivered to the load.

That means almost all the cheap controllers (that are often just labeled "PWM") will have a small dumb IC that's been wired to run at a *fixed* power point.  They do "okay" if you connect a panel with a matching Vmpp. (maximum power point voltage)  A lower voltage (or shaded, or badly angled) panel probably won't work at all, and a higher voltage panel will waste power.  But they're like $3 and you get what you pay for.

TL;DR: there isn't any "magic number" to compare a MPPT controller with a "PWM" controller.  Under the right conditions they could be almost identical.  But in other situations they could be literally ANY difference between them.  80%, 10%, whatever.  It depends on how well-matched the controller is with the panels. (and also solar conditions, battery charge, etc)  MPPT controllers will always cost more and be more efficient, but just how much is *very* dependent on your situation.  (in my earlier post I mentioned a "PWM" controller that came with a panel - so the controller was "matched" to the panel and with good sun it's actually pretty efficient)