The ProtoDev 28 and ProtoDev 40 are essentially the same PCB. The differentiating factor is the IC holder on the PCB. Generally however, for ease of manufacturing the 28 and 40 pin IC holders have been placed on the same PCB. What this means is that the PCB can host both a 28 and 40 PIN device. For example the PIC16F882 and the PIC16F887 could be hosted on the same PCB (not at the same time).
Other than that the ProtoDev 28 and ProtoDev 40 offer very similar features. They both come with LEDs, Push buttons, on board power supply and MCP2200 USB to RS232 communications interface. A prototyping area is also built into the PCB for adding additional components.
For more information, please see the following link: ProtoDev 40