If you are very sure about starting your own firm, then Big 4 has limited attractiveness compared to a med size firm for the following reasons:
1) Big 4 would only give you exposure to one area - mostly audit (either stat or internal) based on which group you get aborbed in and you would end up doing 3-4 main client in all your three years.
Compared to the this a med size firm would give you exposure to more areas of practice.
2) You can use the relationships you make in the med-small firm when you start your own firm but thats not the case with BIg 4 as the clients are simply too big.
3) You get more time networking compared to doing artcles with a Big 4.
However having said this if you are not so sure about practice and want to keep the option open certainly go for a Big 4.