Batch upload documents or one at a time

I am designing an application service for a mobile phone. The user must submit a bunch of photographs and documents "the application".

The way I have designed it, documents are stored locally then uploaded all at once at the end.

But the existing service uploads as the user goes along, one at a time.

Pros and cons:

Has anyone got any evidence of what might work better? A big draw back of the existing API is user cannot preview files once uploaded.