About difference in levels, at least for phonak they say - the more difficult environment, the higher level you need.
My interpretation based on info I’ve gathered:
30 - you’re basically inside in quiet or in predictable simple environment most of the time
50 - quiet but also some street noise, eg you might need better noise reduction
70 - better speech in noise eg in restaurants
90 - better speech in speech babble eg in networking events or in big rooms with echo, or hospital corridors.
30 and 50 don’t have 2 mics per aid but only one, so bigger ones have better results in directionality, especially when they need to reduce noise from one direction.
With a disclaimer, which you already know, no HA gives you what you’ve lost.
Smallest unitron really sounds similar to marvel 90 when I’m at home, in quiet, watching tv. Or better said, both aren’t anything special.
However, I heavily enjoy direct streaming and it definitely improved my quality of life, be it directly from tv, phone, external mics.
I did try 70, 90 and some baby unitron, lowest level.
Since I tried two only in 90 version, I can’t vouch for how exactly good 2x 70 would be. But two finally give me sound boost I lack. However, only one aid on my bad ear, not that useful. However, my case is that HAs are expected to be of limited use anyway, so whatever I can get out of it, it’s great.
What I can say is that with these two 90s I finally ain’t tired or exhausted off the istening. Although I still like silence and my tinnitus came back hard and loud when I remove them. I decided for top level because I will definitely be in difficult listening situations and any help is appreciated.
So I’d recommend assess what are the 90-95% of the environment you expect to be in in next 5-6 years and then compare two around that, eg 30/50 or 50/70 or 70/90. My fitter said that even though majority of his clients are older, and they mostly want to try 90,they end up with 70 because they just can’t see the difference between them while testing them in their expected environments. I can’t test the networking but could test the hospital hallways, and I definitely have easier time than I ever had with my old widex. However I didn’t test 2x 70 in exactly the same environment. One 70 wasn’t anything special.
But, I can afford 90s so I stopped thinking it over all the time. So yeah, see your budget and work from there, that’s also an option.
If they’re set properly, all will work the same in quiet environments, having 6 or 26 channels.
What’s proper, check best practices summarised by dr cliff