I don't have a phone sensor or any device that measures HRV. I have a plain smart phone and an old-fashioned wrist watch! So I have to buy something to measure HRV and I chose the corsense device because it seems to be the most accurate and the easiest to use, just like a pulse oximeter.
I can honestly say that I've used my Corsense for the last couple years and I am so completely in love with it. I had to pay way more than what Americans pay for it retail as well since I had to pay in Canadian dollars for the device and for international shipping and it was still worth the price for me.
I used the Elite HRV app with a heart rate strap for years before I was able to finally save up for a Corsense and even with how fiddly the heart rate strap was it was still a game changer for me when it came to pacing.
I've also used a Fitbit for at least five years and now just recently got a Garmin watch. While I've found both useful for keeping track of steps and regular HR pacing I have to admit now looking back on the last five years of sleep tracking data, at the end of the day all that sleep data really ended up being pretty useless to me. The thing that has been the most useful to me has been looking at the data from Morning Reading with EliteHRV. I know everyone else's millege will vary though.
I do feel the need to warn people that it takes a long time of data tracking to actually be able to see patterns in your data that you will find useful to recovery. It took me at least a year of taking Morning Readings with the EliteHRV app through trial and error to figure out what I needed to do to make sure that my reading was in the green more often. The app can only tell you when you are heading towards a crash and when you need to rest more. It doesn't tell you what habits that you need to do on a daily basis in order to get better readings.
While tracking heart rate variability stopped me from heading into hardcore crashes within the first month of using it because I could see from my Morning Readings that it was coming --- the real benefits that come from it can take years and years. I've been tracking for around 5 years now and I am still learning things through trial and error.
Sorry if I ended up writing a novel there but I didn't want you to get the idea that tracking heart rate variability was any sort of 'quick fix'. It is long and hard constant experiment to try to figure out what techniques are working when it comes to pacing. The reason it has been a game changer for me is now I have real mathematical numbers to help me figure out what is going on instead of just hoping to make a best guess when it comes to how I am feeling.
Edited to add: I've also tried using the Welltory app in the past and didn't find its data really useful to me at all and ended up deleting it.
Double edited to add: I also forgot to say that using the EliteHRV app + Corsense has very recently allowed me to start doing body weight exercises without crashing because I now have a much more solid understanding of my symptom patterns. Much to my complete amazement I've slowly worked my way up to doing 10 repetitions of an exercise. Something that if someone would of told me 5 years ago that I'd ever be able to do I'd would of thought they had lost their mind. I know 10 repetitions may not seem a lot for some people but to me and how often I've been bedridden off and on, it feels like a miracle.