Robert,
Your choice will depend mostly on what tou can locate for an interconnect board. For the 2143 you can use an ICM-20100 which is relatively inexpensive even new. The 1842 will need an ICM-1900 which is pricier new, but probably more easily available used. The 1842 will also have analog inputs, which are nice, but not necessary for most people. My personal choice is the 21x3 series with a RIO connected via Modbus to handle extra I/O both digital and analog.
Kenny