Good question! Usually the bigger branded firms hiring someone at experienced hire level (ie. with a few years' experience) will be looking for candidates that have prior consulting experience and some sector or functional specialism. So if with a smaller firm you can become a subject expert on public sector reform or the telco industry then you should find it easy to switch to a major branded firm at a later date.
The other option joining a smaller firm would keep open for you is the possibility of going to do an MBA at a top business school. This would then be a door-opener for you to get into a firm like McKinsey or Bain at a later date.
Hope that helps and best of luck making your choice! Tony