For those of you that have tried these, I'm wondering if the stem (or even bar) mount is preferrable over the top tube mount? Seems like having the top tube mount means you have to look further back (and down) to glance at the screen.
I regularly use a stem-mounted phone mount for navigation on my tourer, and occasionally on my hybrid. I have tried my phone in a toptube bag, and don't get on with either the bag aspect or the position.
One benefit I find is that mounting it further forwards means I can mount it more upright, which shields the screen a bit against both rain (waterproof phone) and sun. I've just made a little wind/rain shield for my stem mount, which I'll take on its first long ride tonight. It would be nice if someone made something like this.
On the MTB I occasionally use the toptube bag, but I'm more worried about knocks and more willing to stop for the map, and it's easier to move the toptube bag between bikes than the customised stem mount.
Edit: some pictures of the mount I use, more details on the windshield.
It did a good job of keeping the rain off the screen while moving, but I didn't have to navigate stop-start traffic in the rain so that's still untested.
This is about what I see when riding (if on the drops I can still see enough of the map, but not my stats). The toptube bag holds snacks, keys, and battery pack if needed, as well as the cue sheet/backup navigation.
The mount from the side. It's a cheap one from Tiger (emergency purchase), with the smoked Perspex windshield hot-glued on (there's not enough plastic on the mount to screw into without killing the adjustment). It needs a bit of anti-vibration (more hot glue, and a strip of old inner tube as a rubber band). Ideally I'd have got my hands on some thinner Perspex (this is 3mm, and a bit hard to bend cleanly using just a heat gun and scrapwood), but it wasn't available in time.
Update: That particular plastic phone mount didn't last very long after those shots were takendue to a broken adjustment mechanism. Its replacement couldn't take the weight of my phone, which is essentially armoured - just as well as it hit the road at 30 km/h. I'm currently using a metal phone mount on the aero bars and need to remake the wind/rain shield to fit it.
The most solidly locked-on setup that I know of is the Quadlock range. If they make.a specific case for your phone, then that's the best way to go, but otherwise you can attach the mounting kit to the back of your phone or current case. They do small mounts that will fit on your stem or handlebar, as well as an out-front mount that will be the easiest to use while riding.
I use a cycle computer rather than a phone, but the principle's the same. I have out-front mounts on all of my bikes and wouldn't use anything else. You can just glance down at your screen without having to take your eyes off the road for more than a second. If you need to press anything while riding, then you're able to get your hand back on the handlebar much more quickly if you need full control in a hurry (unseen pothole or animal running onto the road etc.).
I'm using one of those and it's working well, although not perfect if you want to interact with your phone while riding (not advisable in any case). It protects well against the vibration and elements and has protected my phone in a crash (although the bag itself had to be replaced). Good if you want to set some display and forget it while riding. Not the prettiest setup but functional. https://www.amazon.ca/RockBros-Handlebar-Waterproof-Cellphone-Sensitive/dp/B07B9QCD6F
I have used both, I've had a small top tube mounted holder with a space for tools and spare inner tube which is handy. I prefer the angle and look of a stem mounted holder though. Be warned, I first used a twist lock phone mount. I bought a good quality metal stem mount and then a 3m sticker based twist adapter that I stuck to my phone case. After around 3 months the stickers plastic mounting points had worn down and hitting a bump in the road my phone fell off and bounced into the undergrowth. Thankfully it was fine and I have since upgraded to a quadlock system which I hope is more resilient.