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.