In my opinion, it doesn't matter whether the CA firm you are going to join is reputed or madium/small.
The thing which matters is whether you adjust into that firm.
For example: If you are doing articleship from a medium or small firm then it may give you personal interaction with your principal, but it is not sure that overall practical exposure will be provided by him.
If overall exposure is also there then even you may for some reasons may not feel comfortable there.
On the other hand if you are in a reputed firm then it may be possible that personal interaction is not there, but can have that by showing your hardwork to your principle. And other thing that if the firm is reputed then other than just a particular field there is a great possibility that it also has overall exposures.
So it depends on how you adjust and how is your attitude towards it !!