The Prosoft modules are the "safe bet" because they will handle 99% of the functions you throw at them and have the widest installed base and support.
If your application is cost-sensitive, check out some of the other suggestions.
I am posting this followup to mention that you cannot put any modules other than ControlLogix CPUs, ControlNet and EtherNet/IP modules, and the Redundancy modules into a Redundancy chassis.
Because Prosoft modules use backplane I/O connections, they cannot be placed in the Redundancy chassis with the controllers; you would have to place them in a remote chassis connected over ControlNet.
Also consider this issue: if the Redundant controllers switch over, the 1756-ENBT or -EN2T EtherNet/IP modules swap IP addresses. Until your Ethernet switches refresh their MAC ID tables, all connections to the Ethernet modules will be broken. What effect will that have on your Modbus connectivity or a standalone gateway device ? If you can wait for a TCP timeout or for whatever time it takes for the switches to rebuild their tables, you're OK. If not, you should consider a solution that does not rely on constant TCP connectivity to the ControlLogix.