For me, Balmorhea was always an example of shoegazing. But now Wikipedia says, its post-rock. (https://en.wikipedia.org/wiki/Balmorhea_(band))
What is the differentiating aspect between these genres, then?
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The terms we use to define musical genres are labels that can be useful in order to introduce distinctions and refer to subsets of a whole, but they remains just labels. In particular, they show properties that a rigorous subdivision would not have: they can overlap, they often are blurry in meaning, and so on. Assuming there's a difference between genre A and genre B, where to draw the line? This is the kind of difficult questions which are often solved by acknowledging their uselessness.
Post-rock is maybe one of the less precise terms you can think of: its meaning really depends on the style of the first bands that have been thus defined (i think about Slint and Gastr del Sol etc), but has then been used in order to define a huge number of (often very different sounding) bands. Compare, say, Polvo with Giraffes?Giraffes! or Balmorhea themselves.
Shoegaze, at least, can be pinpointed in a rather precise way: it is characterized by huge amount of guitar effect (distortions + deleys + reverbs and a lot of feedback), leading to a saturated, overwhelming sound, which is most often associated with dreamy and ethereal singing. Some say that 'shoegazing' refers to the act of looking towards your shoes during your playing (in order to manage the multitude of stompboxes and effects etc). Balmorhea does not sound very shoegaze to me (didn't know the band until now, but I'm listening to some pieces in order to get an idea). Prototypal shoegaze bands are My Bloody Valentine, Ride, Lush among others.
So, cutting a long story short: you question has no precise answer. In the particular case you are referring to, I'd say that Balmorhea play a kind of acoustic, minimal post-rock and I wouldn't consider them to make shoegaze in a strict sense.
Personally, when I read a post-* label, I tend to think of them in terms of time periods, like post-modern = after Modernism, and a general sense of transcending strict styles.
Post-rock to me is sort of an after Beatles, cusp-of-punk time frame (Talking Heads are a common reference point) where mixing traditional rock with outside styles like reggae or African music, or non-pop elements like Minimalism becomes de rigueur. Music lacking such appropriations or being stylistically pure (ex. 1980's hair metal) is unsophisticated by comparison.
I may be overstating things, but stylistic purity is insular, you need to be a member of the group to really get it, but post-isms are more outward facing, you need to understand diverse, external elements to get the sophistication. The problem is these post-isms start to take on their own sense of being a genre and you end up with a problematic label. One that can refer simply to a time period or a style characterized by appropriation.
To me Shoegaze it pretty stylistically pure. Only fans of the various groups and sub-genres can tell things apart. Admittedly, I don't know much more than Jesus and Mary Chain and My Bloody Valentine, but from what I have heard most of the stuff is conforming to a style. It's post-rock only in the sense of time period - it's after Elvis and Led Zeppelin, etc. - but not transcending style and appropriating from diverse sources.