The first one.
6-drive hot-swap backplane firmware for S3420GP
Supports the Intel® Server Boards S3420GPLX or S3420GPLC server boards in the Intel® Server Chassis SC5600/SC5299 family
The second one doesn't work with the S3420GP, because it is intended for a different family of boards:
6 Drive Bay Hot Swap Expander Controller AXX6DRV3GEXP Firmware
This download contains Intel® Hot-swap Backplane AXX6DRV3GEXP firmware for EFI to use with the Intel® Server Chassis SC5600 SC5650, Intel® Server Board S5520HC S5500HCV S5520HCT S5500BC, Intel® Workstation Board S5520SC.
Note that there is no S3420GP of any kind in that second list.