c3.8xlarge probably, mainly because of a good mix of high performance CPU’s, good amount of memory, SSD storage and 10 GbE.
For better value though, I would load balance between a number of cheaper instances (like the c1.medium) instead.
A c1.medium instance comes at 1/16 the price of a c3.8xlarge, but it will allow for over half the througput (~0,95 Gbps) of the expensive c3.8xlarge 10 GbE instance (~1,7 Gbps).
For the saved files, I would script those to be moved to a S3 bucket after ended broadcasts.