They're pretty much synonyms, but I would argue that the longer word is actually more readily understood by the general populace (at least in America), despite what NGrams may tell you.
First of all, pretension is a more bookish term, so it would not surprisingly show up more in books. That does not mean it shows up more in speech or casual writing (especially online writing).
Second, pretentiousness is a noun made from the adjective pretentious, which virtually everyone knows and understands. You will hear the word used everywhere from literary soirees to shopping mall get-togethers to trailer-park barbecues.
Now, it is likely that most people would understand pretension if they stopped to think about it. They just wouldn't say it.
All right, having said that I am obliged to point out that pretension is much the better word to use with a complement. For example:
My wife can't stand her pretensions to culture.
You can't really say "pretentiousness to culture"; it just doesn't work.
And by the way, you won't hear the phrase "pretensions to culture" in a trailer park, unless said park caters to graduate students.
Extemporaneous, as its roots indicate, is a response that is made up on the spur of the moment. Note the operative words made up, and moment. (Syn.: extemporaneous, extemporary, extempore, impromptu, offhand, unrehearsed, unpremeditated, ad-lib) etymonline: "without preparation, without time to prepare,"
A spontaneous response is involuntary, one is aware of it and 'lets it take control' and not try to make up an alternate response. (Syn.: spontaneous, impulsive, instinctive, involuntary, automatic) etymonline: "of one's own accord, willingly;"
An impulsive response is an overreaching reaction that leaves no chance for external control. etymonline: "rash, characterized by impulses,"
You are asked an unexpected question. You decide to answer it. Quickly think up something nice to say, and there you are with an extemporaneous response.
You are asked an unexpected question. You get a gut-feeling that the response ought to be a certain 'this.' Should you use the gut-feeling? Should you suppress it and think again before responding? The choice is yours. You can chose to be spontaneous if you like.
You are asked an unexpected question. And before you know it, you have uttered something in response. You find yourself in a situation where you were not in control of your reaction. However, you understand that the utterance was prompted by the nature of the question. Some of the times, it is possible to be aware or and in control of one's impulsive reactions. However, generally they are highly autonomous. Like when you scratch yourself in response to a bug bite! (Kids do it, grown ups don't? Maturity, control.)
Best Answer
I think the dictionary definitions you are using are a bit misleading in terms of the way we tend to use these words.
Emotional does not necessarily imply any level of intensity, in my opinion. Rather it indicates that you are referring to something that pertains to the emotions, the feelings, the non-intellectual responses to external stimuli. The intensity of the response may be mild, severe, or anything in between.
Visceral, on the other hand, although it also works on a non-intellectual level, is often regarded almost as if it is not connected with the emotions, because it has the connotation of arising from a physical reaction, commonly called a "gut" reaction, a response one feels typically within the abdominal area (commonly: "the stomach"). This is usually felt to be a more powerful reaction, and so we tend to reserve "visceral" for those reactions which are generated by things that hit us so hard that it almost feels as if we have been punched in the stomach.