>>What will I be “missing” by not getting the M80s?
Let's talk about the differences which can be described by a frequency response spec first. Since the M50 and the M80 have the same tweeter, the high frequency response is going to be about the same. Low frequency is a different story -- you can see that the M80 extends 4-5 Hz deeper than the M50. Both speakers go sufficiently deep that you won't notice a big difference on most music, but on any music where the low bass is carrying any kind of tune the M80 will go down to lower notes before the volume starts to fall off noticeably.
Having said all that, the biggest differences between M50 and M80 don't show up on the frequency response specs, and some of them are hard to find on the specs at all.
The two main differences are that (a) the M80 uses a dedicated midrange speaker while the M50 does not, and (b) the M80 doubles up the midrange and tweeter, and has a larger cabinet with more port volume.
The dedicated midrange makes for a slightly flatter frequency response in the 1-5 KHz range -- M80 is essentially flat while the M50 has a slight dip in the off-axis response. Since the off-axis sound bounces off the side walls and contributes to what you hear, the result is that sounds in the vocal range sound a bit more "laid back" on the M50 while they are a bit more "up front" (ie a bit louder and sound closer) on the M80. A +/-3dB frequency response spec won't catch this, and a tighter spec would probably show other deviations first. You really need to look at a graph or listen to the speaker to hear this difference.
Secondly, the extra drivers and larger cabinet/ports on the M80 mean that (a) you get a bit more sound for the same amount of power from M80 over M50, and (b) the M80 can handle a lot more power than the M50 without sounding strained. These differences show up in the "sensitivity" and "power handling" specs, although be careful because these specs are mostly useful within a single vendor's products and very hard to compare between vendors.
Impedence also makes a difference but that is really hard to explain so I won't try right now
Bottom line is that the frequency response spec is useful in two ways -- if the response is within 3 decibels (dB) across the range then the speaker is probably reasonably accurate (if the spec doesn't say +/- xxx dB then be suspicious), and the range itself gives you an idea whether you are talking about a bookshelf-type response curve (maybe 60-80 Hz at the bottom) or a seriously full-range speaker which normally has to be larger to give the bass extension. You can, of course, get bookshelf speakers with deeper bass response but generally they give up something else in exchange, typically power handling or sensitivity.
