I was a senior manager in Accenture's consulting practice (left in 2002), and still work in the consulting industry. You could probably contact an Accenture recruiter to get an "official" answer. The following is just my opinion.

If you currently have what it takes to get hired by Accenture, doing a couple of years at a startup is probably not going to hurt your chances of being hired later, but it probably won't help you get much of a "jump start". The early years at Accenture focus on "acculturating" you to the consulting life (and specifically in how Accenture does things). Most of the big consultancies follow this approach, as a consistent approach is a key success factor for them (much the same reason why the military gives everyone the same basic training). In some cases the big consultancies will hire someone in at a senior level, but typically this is for people with industry-recognized expertise.

BTW, my advice would be to take the start-up role, unless you need the steady income of a corporate job (e.g., you already have a family to support). You won't have the same kind of freedom in the future. I still wish I'd tried to interview at Microsoft when I graduated in '85, rather than taking the safe path in IT.

Thanks for the info. The "acculturation" part was particularly enlightening

I hope that doesn't sound too sinister. My wife used to refer to it as "the brainwashing". The reality is that Accenture needs to quickly bring together large teams of people who have not previously worked together, and make them productive as quickly as possible. One way to facilitate that is to ensure that they have a common language and understanding of how projects are run.

I don't know if they still do it, but throughout my tenure at Accenture new hires were sent for an initial training course at the firm's St. Charles campus. The course was traditionally 3 weeks long, and in addition to teaching basic programming to those who had no prior experience (many Accenture hires are not CS or engineering students), it gave new staff some experience with how Accenture runs projects. After the course, new "analysts" are assigned to their first project. The first couple of years as an analyst tend to be "in the trenches" type work, as people don't really have much industry experience. After a couple of years as an analyst you become a "consultant", and start to get more authority and responsibility. It typically takes about 5 years to make it to "manager", at which point you start to get real authority within project teams. From there, career path's vary. I don't know what the current stats are, but during my tenure I would estimate that about 1 out of every 10 hires actually stayed with the firm to the senior manager level.