They are so very similar, differing mainly in the direction of the CD and RI signals. They have similar functions controlled by certain signals, but which signal implements a particular control differs between them. My vote would go to the 2150, since for a receive-only application, the signal functions make more sense to me in that context.

If you are in control of the software on both sides of the link and don't need to operate with other IrDA devices "in view", you can simplify things a bit by using only an infrared encoder/decoder device (MPC2120 or many others exist) and run the link in a truly "raw" mode. This was briefly discussed awhile back in the thread http://www.keil.com/discuss/docs/thread1283.htm.