I think it depends on the context and the user.

Bitcoin-er like you said is the most commonly used by people when trying to depict someone who invests in bitcoin or an enthusiast, so I b lie e majority would prefer using Bitcoin-er.

Bitcoin-ist on the other hand, I believe sounds a lot more professional, although not commonly used but way more cooler and professional.

The suffix "ology" is mostly used when denoting a field of study, I believe the term "Bitcoin-ology" could be used in state the study of Bitcoin.
Although it doesn't sound as cool as bitcoin-ist and bitcoin-er but I think it'll do just fine.

Everyone has the right to be called in any call sign as a bitcoin holder or investor, and everyone can claim themselves as a bitcoin holder or investors, but of course it is not easy for everyone to actually do good in bitcoin because it is a very important for a so called bitcoiner to have the proper knowledge and skills in order to earn or have a profit in doing bitcoin, anyway no matter what called or call sign a bitcoiner in the end the most important is that everyome will know that you do bitcoin and it really helps you so that many people will become more interested in bitcoin and be attracted to do bitcoin. But for me Its better or sounds better to hear "bitcoiner" than "bitcoinist" its more pleasant to hear and more good as a title or to be called, and it will be an honor to be called one.

I read the best reply from the starting comments on this post and I must say that while bitcoiner sounds like a professional point of view or should I say one that invest or trade Bitcoin, a bitcoinist sounds more specific like an investor whose only interest of investment in cryptocurrency is Bitcoin, not shitcoin, not altcoins.
Just like we have extremist or terrorist or fascist, a bitcoinist is a die hard investor with absolute believe in the existence and returns of Bitcoin.

Bitcoin is not up to two decades and English language has been there for many years, so I am not surprised that English dictionary don't have provision for bitcoinist or bitcoiner. Bitcoin took the world by surprise and till tomorrow may people are still trying to figure it out. But with regular and continuous usage of the word bitcoiners for some who is bitcoin enthusiast and bitcoinist for the professional study of bitcoin, won't be bad. If it's continually use henceforth, it can be globally adopted and hopefully one day English dictionary can recognize it and put out it into their revised edition. Those words ain't really that bad, they actually sounds sweet in the ear when you listen to someone pronouncing it.

If we're choosing between Bitcoiner and Bitcoinist, I'm definitely on the side of the first option. That's because -er and -or usually signify that it's a person that's doing something: employer, buyer, investor, etc. On the other hand, -ist has a lot of negative connotations that are related to accusations of people wronging others on the basis of certain social identities: racist, sexist, ageist, etc.
I can see that there are different arguments in the thread (like physicist, archaeologist, etc.), though, and it's also a fair point.
As for the study, I agree with those who are suggesting Bitcoinology.

There was a thread that I have made before about Certified Bitcoin Professional.
This can be another call for the right name to be used if someone is addressing a person who's involved in Bitcoin but not all of us are professional and this kind of title requires to pass on an assessment or exam.