I’ve used both Phonak Marvels (universal) and Oticon Opn S1s (MFi and universal / ConnectClip).
The Phonak BT implementation is based on the BT “classic” Hands Free Profile (HFP) and Advanced Audio Distribution Profile (A2DP). HFP is used for two-way (low data-rate monaural). A2DP is used for one-way (high data-rate stereo). Implied by the name “classic”, these profiles were developed in early on in the development of BT standards, long before anyone thought hearing aids would ever support BT. Therefore, they are both designed around one hearing device being connected to the mobile. This requires one device (left or right) to transmit to the other device. In the case of hearing aids, the hearing aids are separated by the wearer’s head, so they work best when there are surfaces that can reflect the signal, since a head is a good attenuator of a high frequency signal like BT. There is also some latency introduced by this design.
The MFi and ASHA approaches are completely new implementations, based on BT low energy. BT LE doesn’t support the BT classic HFP or A2DP profiles and replacement profiles don’t exist (yet), so Apple and Google designed their own one-way profile (high data-rate stereo) which is similar to the BT classic A2DP profile, but streams directly to both hearing aids simultaneously. I’ve never used ASHA, but my understanding is that it works identically to MFi. MFi and ASHA are interim standards and they will eventually be replaced by profiles that will be built-into the BT standards. Neither MFi, nor ASHA support a microphone. IMO, this is not a loss, because you can easily use the microphone from the mobile for the microphone. The audio quality is much better than the Phonak, anyway.
I started with Phonak and switched to Oticon. My opinions of these are based on my experience. YMMV. There were long and tedious discussions comparing them on this forum.
I switched from Phonak mainly because when they were first introduced, they could only be paired with one device at a time. This was a show stopper for me, because re-pairing required me to reboot my laptop which was very time consuming, due to the number of applications that I use. I also found the connection to be unreliable to the point where I could not depend on them for business meetings, so I had to have a pair of wired headphones on stand-by in case the hearing aids stopped working. It was inconvenient and very embarrassing to disrupt meetings due to the equipment not working. I also received numerous complaints about the quality of my audio, perceived by people that I was communicating with. Also, due to the design, the device that is connected with the mobile not only has to receive from the mobile, but also transmit to the other device, so it uses more power than the other device. Phonak had no fix for these issues, so I trialed Oticon Opns and they worked so that was that. Phonak eventually fixed the single pairing limitation. They can now be paired with two devices. By that time, I had already moved on.
The Opns don’t directly support BC classic, so they require the ConnectClip to connect to a non-iPhone or laptop. While it’s an inconvenience having another device, it works very well, so I don’t mind it at all. In the morning, I turn it on. At the end of the day, I turn it off. Same thing the next day. I don’t remember the last time it dropped the connection. They connect directly to an iPhone using MFi. This generally works well, although occasionally, if I’ve just been in a call using the ConnectClip, it can take a few seconds (or more) for the connection to switch from the ConnectClip to the iPhone. Sometimes, I have to turn BT on the phone on and off to force them to reconnect. While I’m doing this, the audio temporarily goes out through the speaker, then automatically switches to the hearing aids. I don’t miss anything and the caller has no idea it’s happening, so it’s annoying, but not the end of the world.
If you’re only using a mobile, there is no question that the MFi / ASHA approach is better. Eventually, this will be standardized. Battery life and sound quality are both better using BT LE. (I would hope so.) For people who don’t have an iPhone or a newer android phone that supports ASHA, the only choice is BT classic. For these users, the choice is either Marvels (direct) or Opns with a ConnectClip. This is a personal trade-off. I had a miserable experience with the Marvels, so I don’t mind using the ConnectClip. (The iPhone is my primary device, so I’m using MFi most of the time when I’m not on my laptop.)