In short: both are speculative and risky.
I'm not going to argue which one is best, they could both be trash for us common folks without a background in finance. Still, there are a few things that makes them stand apart in terms of predictability and risks if you are a startup employee and you are torn between investing in your startup or in a cryptocurrency portfolio.
In terms of predictability, cryptocurrencies wins by far. If you are nose deep into social medias, you have learned how to read candlestick charts and are following some prominent crypto investors: it's relatively easy to see upward or downward trends. There are obviously some surprises, markets can behave erratically but these events could be managed. As long as you don't buy at highs and sell at lows or invest into a dubious coin with an overhyped ICO, you are guaranteed to make money. Multiplication factor in the X0 or X00 in the rare occasions you are lucky and decided to go all in into a crypto that went through the roof, or a long time holder.
- Bitcoin (and all cryptos) current dip was highly predictable, because of the SegWit activation (as of July 2017) and the potential of a split.
- Ethereum going almost 10X was highly predictable after the ICO boom around March/April. You could clearly see an upward trend with bullish signals on candle-stick charts.
Cryptocurrency value is often overblown, with people manipulating the market for their own gain, but at least it is (somewhat) predictable and big trends could be seen if you keep yourself informed with new product launch and good/bad news in the crypto market.
For startups, as en employee, it's often very hard (impossible?) to get the true grasp of a company's common stock value. Informations are siloed higher up and/or highly distorted amongst employees which results in a very low ability to foresee trends. Yet you are expected to buy back your stocks to show your trust into the company, sometimes with no clear signs of healthy finances. It could be even worse: you are withheld information on the company's health.
Some startups are clearly winners, and you can see them from afar but because the time scale is stretched to years to see a successful IPO, it's also risky as an investment. In case of a failure, not only did you waste money (and probably worked for free because of tax implications), but also your time and your ability to invest in a company that could have proved profitable.
Both are risky, but I'm going to argue that the crypto market is less risky than the startup stock market as of now.
The strike price to buy back stocks is often in the order of 5/10$ if you are not amongst the early employees. Thus you'll have to spend many thousands, tens of thousands of dollars to exercise them all.
Cryptocurrencies are still in a time where you can invest a few cents on a coin with predictable signs of potential and x100 returns. So the risk in term of up-front investment is lower for Cryptocurrencies than it is for common stocks.
Startups are also very demanding, you need to invest 100% of yourself in it. You can often compare them to relationships: a first phase of idealization, where everything is going well and you experience high productivity throughput. Then a drama happens, and you want to leave: but you are bound with a stupid clause or stock option exercise window that makes you reconsider and annihilate the chances of potentially finding a good company fit with a higher potential of doing an IPO.
There is less sink cost with cryptocurrencies, lower time frames to make money in crypto than there is for startup common stocks.
I would say the biggest risk with cryptocurrencies is the potential for losing everything in a hack. Although you shouldn't be keeping your coins in an exchange platform anyways. You should consider transferring these coins onto a secured piece of hardware like the Ledger Nano S, Trezor, etc.
I'm not even tackling the issues around taxes, but crypto wins depending on where you live (Denmark and Japan: 0%!), most countries have very permissive tax systems for cryptocurrencies which is not the case for common stock options.
Again it's not an advice to invest in crypto, the point is: startup common stocks are overhyped and have no real advantage for common workers (if you are not an early employee or a founder). They are interesting for early employees and upper management though with potential for big returns if the company does well. Even with obvious volatility and flaws and bad actors manipulating the market, it might be better to invest in crypto than in common stocks right now (or invest in stocks like Amazon, Alibaba, companies already established or with a big potential, etc..). I'm no expert, but this is my take for having experienced both. This is probably not going to last long for crypto, once the market matures and actors become more rational with their investment, the potential for big returns will eventually be lower.
Although if I had the ability to talk to my past self: I would advise to take the higher salary with no stock options and invest in crypto instead.