I've been looking to buy the BH-121. I didn't even know about the BH-221.
Saying that, the BH-121 is a newer model and is designed to work with the newer WPs and has all the latest things you'd expect. It of course lacks the FM radio and display but that all depends how important that is to you.
From a personal perspective I'd rather get the latest and ensure compatibility over possible disappointment. One thing I read about was using this device to hook up to my car stereo via the AUX jack which is a neat idea.