In my opinion, if you get a firm which is actually Mid Size you should prefer that over Big 4.
But remeber firm should actually be Mid Size, many firms are called mid size but they are necessarily not so.
Poeple some times call a firm mid size just going by the fact that it has many clients.
See, a CA FIrm might have many Pvt Ltd's as clients. But this Pvt Ltd's may be so small that they are run like proprietorship. I wouldn't call such firms as MId Size.
But there are many CA firms which have Mid Size Corporate clients. If you get to work on this clients, in my opinion you get more learning.
But dont go for a small firm, go for Big 4 if you cant get a Mid Size firm.
However, the point you have raised is such that everybody will have different opinion.