For the plan that Daisy mentioned, a 15% discount on a 6-month period where you get the lower of the beginning price and the end price, the potential gain in share price is in fact taxed as capital gains.
Eg, stock starts at $100 and ends at $120, you get it for $85, you are taxed on $15 as income and $20 as capital gains. The capital gains are short-term if you sell within two years of issuance, I believe, and long-term if you sell after that. (The two year thing is slightly more complicated but it can be summarized as that, in most normal cases.)
At a normal engineer's salary in a high-paid area, that's 28% today - 28% on $15 is $4.20, and 28% on $20 is $5.60, totaling $9.80. If you wait two years, assuming no change from the $120, that ends up being $3, for a total of $7.20. In other words, if you sell immediately you get the difference ($120 less $85 is $35) less taxes ($35 less $9.80 is $25.20). In this synthetic example, that's a gain of almost 30%. If you wait, you get $27.80 which is just under 33%. However if you had sold immediately ($25.20) and invested in an index fund, you should be up to a bit over the $27.80 usually...
So let's consider what can actually happen between purchase and sale.
- Large drop
- Small drop
- Flat
- Small gain
- Large gain
Let's see. If there's a drop, then you shoulda paid the taxes. You might think that if the drop is small, or is flat, or is a slight gain, you won - except you most likely didn't because investing into an index fund would have given a better return. The only time you really win by waiting is if the stock goes up well above the market.
So, do you want to bet that the stock of your company will go up significantly? There's nothing wrong with that belief; I mean, you work for the company, you might be loyal to it... if so, then feel free to keep it. However, you will almost always be better off selling immediately and reinvesting than waiting two years.