I would actually argue the opposite: the IBM job (or some equivalent big-company job) will always be there. The startup opportunity at [name your startup here] is only available now.

Big company jobs tend to be alike in many ways (due to their size), whereas startups vary so significantly that it's deceptive to think of all startups as interchangeable.

So yes, there will always be some startup opportunity available, but if you know that there are good people working at this one, or if they're working on interesting things, then I think it's a better option.

Companies like IBM tend to be most interested in finding high-ranking college grads to bring in. Additionally many startups appreciate and value someone who has solid experience at a large organization, coupled with a drive to do more.

Doing it the other way around (try a startup, then go to BigCo) is less optimal.

As one data point against your statement, from PG's "Hiring is Obsolete":

I asked managers at Yahoo, Google, Amazon, Cisco and Microsoft how they'd feel about two candidates, both 24, with equal ability, one who'd tried to start a startup that tanked, and another who'd spent the two years since college working as a developer at a big company. Every one responded that they'd prefer the guy who'd tried to start his own company.

http://www.paulgraham.com/hiring.html

Don't believe the OP was starting a company, he's coming in as an employee. Big difference.

"The IBM job may or may not be there in another 5 years. There will always be some startup opportunity available."

Another good point, between this and what ken said I think I'm leaning IBM now.