It depends a lot on how you expect people to come to your site and the cost of losing them versus the cost of the better domain. If you run a TV ad for example.co , you can expect an order of magnitude more people to visit example.com instead.
We're also probably at the point now, where even if your alt domain service becomes highly successful, there's a good chance the .com won't be available no matter what (short of a million dollar purchase, either due to a serious business owning it, or ransom).