I’ve participated in lots of discussions about open fit vs. closed fit over the past few years. Here are some links to a few of those.
Basically, with an open fit (if it fits your hearing loss), your own voice won’t sound occluded, and you’ll hear low-frequency sounds “naturally.” But speech may not sound as crisp as it might be with a more occluded fit. You’ll have trouble controlling noise going straight to your eardrums and bypassing the HA filtering/cancellation mechanisms. You might lose too much bass when streaming.
With a closed fit, you may not like the sound of your own voice (too loud, stuffy, etc.) but you can titrate the size of the vent with Select-A-Vent if there is room to accommodate it in your custom molds. You’ll have more control over noise since less of it will leak directly to your eardrums. Speech may sound crisper if the shape of your ear canal leads to some cancellation/interference between HA processed sound and sound making its way directly to your eardrums. You’ll get more bass when streaming but perhaps less when listening to external sounds unless compensation in your fit is made.
Here’s a recent discussion in a thread: Resound Omnia vs Phonak Lumity - #42 by jim_lewis (see posts around the linked one).
Here’s an older discussion: Article on open fit vs closed fit started by MDB. And down in the thread, I specifically mention a 2006 Widex paper on the subject. Article on open fit vs closed fit - #10 by jim_lewis
Unfortunately, Widex no longer makes the paper available on its website but it was published in Hearing Review and here’s an https:// link to the version (no longer an insecure HTTP:// link): Fitting Tips: How Do Vents Affect Hearing Aid Performance? | The Hearing Review
It was reading that 2006 paper and posts by @Don, who also believed in occlusive fits, that induced me to try them. If you go to a 1 mm vent size, that reduces the occlusive sound of your voice a bit but gives you most of the noise control and bass amplification you get with a more occluded fit, in my experience. I like the M&RIE receivers (microphone on the receiver in the ear). It really helps with sound localization for me and makes sound in the environment more natural in that regard for me. I couldn’t wear M&RIE receivers if I didn’t opt for an occlusive fit. So for that benefit and getting the most out of Front Focus, it’s worth it to me. One can quickly get used to the sound of one’s own voice with an occlusive fit just as one gets used to more amplified sound than one was used to before wearing HA’s. But, as always, YMMV.