It's not unlike the decision of doing a startup or working for someone else. You can either settle for a typical paycheque, or you can take a gamble, which might see you create a huge hit, or might flop and bring no returns.
I'd argue that you're playing longer odds in the app store. At most startups you'll be pulling down some kind of salary at least. Keep in mind that the median revenue for iOS apps is < $1000 and that at this point you're going to have to invest quite a bit of time to produce something good enough to attract users.
I think a better way to put this is if you want to make GUARANTEED money writing apps... I still think on the whole, its better to release a product yourself than do consulting for someone else.
It's certainly a lot more fun, but if your goal is revenue, particularly as a solo developer, you're going to have to do very well on your own to match what you can easily make as a consultant/employee.
I don't think it has to be either/or. I took about a month off to write my iOS app. While it never made me rich, it still provides a nice income stream that continues today. Now I have my regular job revenue and app sales revenue.
