Both Cuban Son (CS) and Mexican Son (MS) share roots in Spain's Flamenco and Andalusian Folk Music, and in many of Africa's various music traditions, all of them brought from the migration craze of the New World. This influence is reflected everywhere: harmonies, melodies, lyrics, rhythms, instruments. In harmony one example is the Andalusian Cadence, a common Flamenco chord progression that can be found everywhere in both CS and MS (in a Phrygian context for Flamenco, harmonic minor for CS and MS).
Both are heavily based on string instruments, guitar-like instruments particularly. Both are big fans of improvisation in music and lyrics. Both have their own particular dance tradition attached, and in many regions it is the music expected to party to, those parties being part of the tradition as a whole. Both saw birth from a region around the Gulf of Mexico.
One of the big similarities is that both happen around a constant loop and switch between instrumental melody and lyrics (voice melody) sections. Another big one is that both make use of a "declaration-reply" dynamic, albeit in slightly different ways. Both use thirds and sixths extensively to harmonize multiple singing voices. Both are contemporary, and in many senses "brother" traditions that actively inspired each other during their development.
Yet they have a distinct, particular, sound.
One of the prominent rhythmic differences is that most (if not all the traditional ones?) MS songs are in 6/8 (some use both 6/8 and 3/4 at the same time, known as Sesquialtera or Hemiola). In contrast, most CS songs (if not all the traditional ones?) are in 4/4 or 2/2. The clave rhythm is everywhere in CS, while it is rarely found (if at all?) in traditional MS.
Instrumentation is another one of the big differences. Traditionally they are played with different instruments by different ensambles. This difference in instrumentation gives a very particular tone, timbre, and feel to each Son (this also happens regionally, inside CS and MS substyles). The familiar 6-string classical guitar seems to be much more common in CS.
Some instruments specific to traditional CS include:
Some instruments specific to traditional MS include:
Falsetto is widely used in some local styles of MS, this is not the case for CS. Some of the most well known Mariachi songs are MS, but they rarely play CS (because of the expected traditional context). Cuban Bolero found its way to the Son mix, and developed into the Bolero-Son (link in Spanish), while the MS and the Mexican Bolero (which is rooted in the Cuban Bolero) developed much independently, perhaps because of the differences in metric.
Traditional Ensembles Playing Cuban Son Live
Traditional Ensembles Playing Mexican Son Live