I wear the OPN 1 and I would say that the most significant difference between the 3 models is in the area of Speech Understanding as you can see in the link showing the comparison between the 3 models. The heart of the OPN is the OpenSound Navigator which has 3 modules, the Analyze module feeding into the Balance module feeding into the Noise Removal module. The Balance module helps makes speech clearer by attenuating the loudest noise sources placed between speech sources. But not all noise sources have precise locations that can be attenuated by the Balance module, for example, diffused noises or noises directly behind your targeted talker. The Noise Removal module takes it one step further and reduce this residual noise that still taints the targeted speech that has been rebalanced by the rebalanced module. As you can see, the OPN 2 and 3 only operates at 50% power in the Balance module, and in the Noise Removal module, the OPN 1 can be set up to 9dB noise reduction, while the OPN 2 is limited to only 5dB noise reduction and the OPN 3 is crippled to a measly 3dB noise reduction.
So if speech clarity in noisy environments is important to you, then you’ll short change yourself of that goal if you go for the OPN 3. But otherwise, I think the differences in other areas are not as significant as the Speech Clarity/Understanding area.
If you want to explore options for cheaper versions, also check out the Sonic Enchant 100. There’s been a good review on it from a forum member here who wore OPN 1 and was equally impressed with the Sonic Enchant 100.
The OPN 3 is $3,300 on BuyHear while the Sonic Enchant 100 is only $3,200 there. BuyHear may tell you that the Enchant is just a rebranded version of the OPN, but I don’t find any indication of that. While they may share some of the hardware features and have similar specs, it seems to me like the signal processing is different between the 2 brands/models.