magview, because like @CorbLand said its always with you and you don't have to buy a separate phone case every time you update phones like with the ollin system and they charge $40 a piece.
Also, if you qualify for expertvoice you can get all magview products 50% off
I have the MagView and also some cheap plastic thing off Amazon, you've seen the type, the one that's basically a bracket for your whole phone and has thumbscrews to align things.
The MagView is small and light, enough that I can just keep it in my bino harness all the time. So I always have it. But despite folks claiming it doesn't interfere with using your optics, I personally find it uncomfortable to press my eye against it. My binos have rubberized eyecups like most do, so I found a tightness level that just about holds it on but I can remove/attach it without tools.
After that you need the magnetic backing plate for your phone, which I personally find very fiddly to align. I ended up putting a plate on a spare phone case that I use just for this. Finally, I know we all spend a ton on optics but it seems way overpriced to me for what this thing is.
The bracket-style phone mount was its own hassle. I don't know about others but mine mounts right on the eyecup which adds some twist to it. On my spotter that makes it want to change the fine adjust on the eyepiece. It's also more bulky and just as fiddly BUT you do the fiddling ahead of time, once, and you're good to go after that.
If I had to do it all over ahead, I'd keep trying new products, maybe the Carson or something like it. Neither one really nails it for me.