In deciding whether to concentrate on full-time work or business-building, I think a lot about the different characteristics of the income received.
Full-time work is typically a constant $N/year. You (hopefully) receive a payrise each year. I’m going to assume 5%/year is a sustainable increase over the course of a lifetime. You put the difference between your income and expenses into investments (equity), which I’m going to assume earn 7%/year. Your net worth over time will look something like this:

Business-building - specifically, software, music, writing or other royalty-based media production - has different characteristics. In year 1, you are going to have fairly low income. You’ll do some piece of work and it will earn income on its own. This is passive income at this point - you don’t need to do any additional work to maintain that income. Then, in year 2, you’re free to do another piece of work - adding to your passive income. Lather, rinse, repeat. Your net worth will look more like this:

The question for any rational person is over the course of my lifetime, which is going to give me a higher net worth? And that’s a difficult question to answer early on. You’ve got steady but rapid growth versus exponential growth - but that growth factor is unpredictable.
This is the sort of bank balance that a startup company might see as well. There are two critical measures to take off this graph: how far into the red does the company go, and is that growth rate sufficient to make it a worthwhile investment? Many startups never even hit positive growth and will just run out of cash. Someone said that starting a startup was like jumping out of a plane; the ground seems like a long way off, but you’re falling very fast as well, and the question is whether or not you can pull out of the fall in time.
In the same way, my savings are my investment. If I run out of cash, I have to stop. I need to build income before I run out of savings. I don’t particularly want to risk all of my savings on this one venture, which is why I’ve set a one year time limit. If I’m still plummetting towards bankruptcy at the end of the year, I need to reconsider what I’m doing and if it’ll be effective. (Realistically, I’ll be watching this all of the time. One year is the big yay-or-nay decision, though).
If you need to borrow to get capital investment, this is going to make your profitability a whole lot tougher. You need to match your loan rate just to break even, as I found while playing with the spreadsheet model I used for the above graphs. Just as compounding looks like a lovely exponential growth curve, it can drag you into a horrible pit of debt very quickly.
Individual preferences are obviously going to play a big part in this decision as well. If you like to live close to your means and enjoy your money, you’re going to want the ongoing income. If you have dependents or medical bills, you need the constant income. I’m have no dependents and am happy to live a fairly lean existence, so I can choose to take this risk.
An interesting point that I read about today was that most micro-ISVs can’t keep building income forever. The income is not truly passive. Support is a big time sink, and the amount of support increases with the number of users. If you’re earning enough you can hire more people to take over this load - but this is the sort of problem that I’d like to have. If I get there, I won’t be complaining.
Playing with the spreadsheet model is highly educational: using the assumption that salary grows at 5% means that your income under a business also has to grow at at least 5% for it to be worthwhile. If you’re increasing your income by a constant amount each year, this is going to be easy to beat in the beginning and rapidly become impossible.
But, wait - there’s more! Part Two will discuss the two strategies once you retire.