The configuration methods and tools make a difference in the device's ease of use and flexibility.
Red Lion has Crimson 3, which is probably the most flexible way to move data around, but has the steepest learning curve. The virtual HMI is a very useful feature as well.
Anybus Communicator is fairly straightforward, and their documentation in English and German is good. Their footprint is small and their hardware is durable and well engineered.
Spectrum's Universal Gateway uses a browser-based configuration. It's a relatively new product, and the chief application engineer is a Forum member.
Both the Spectrum Universal Gateway and the Real Time Automation gateways benefit from browser-based configuration that contributes to their ease of use.
Prosoft is the gold standard for reliability and backward compatibility, as they've been around the longest. Prosoft Configuration Builder is not modern and polished, but it keeps up with the times and features do get added. Some Prosoft devices can be configured entirely from within the PLC program.
If I was told "connect one Modbus RTU device to ControLogix, but your junior engineer has to do it and you can't help", I would reach for the Spectrum Universal Gateway.
If I was told (which I was, last year) "you need to connect an unknown serial protocol that was reverse-engineered ten years ago on a custom microcontroller, but you have to commit to the platform before you get any details", I would choose Red Lion.