Garmin
Forerunner 255 Series
Long battery; but wrist HR tracking is inconsistent.
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Yes, try just using the H10 with either the Polar Flow or Polar Beat app on your phone to begin with. The one issue you might run into is that your phone is pretty old and might not be able to run the iOS needed for Flow or Beat. Flow needs iOS 17.0 or later. Beat needs iOS 12.0 or later. As far as an actual watch is concerned, there isn’t a lot of difference in features between the Vantage V2 and the Pacer or Pacer Pro, so it really comes down to price. I had the V2 for about three years and it was quite good (I now use the V3). No experience with the Pacers. The Unite is pretty old and the single button interface and lack of built in GPS make it not great for running. Good luck
I think this was true when I got my GritX Pro \~ 2 years ago. It might even still be true with some watch series. Your Vantage V3 is $700 USD. For price, Garmin's Forerunner 970 is comprable at $750. The 970 has maps and has a sapphire lens for $750. If I'm spending $700 on a watch, personally I'd spend the extra $50 for the sapphire...but I'm rough on my watches. Which is why I like the GritX line. But the value here is pretty terrible now IMO. X2 Pro is $1000. Same price as the Fenix 8, which I think is probably a better watch. Basically every review I saw for the X2pro when it came out was "really nice watch...but the pricing strategy here is very aggressive making it a poor value". X2 is $800 in the US. Same price as the Fenix E (not sapphire though). And more expensive than any of the Forerunners (970 is $750). I think I'd take the 970 over the X2 if I was going to spend that much (and I don't want to spend that much). If the X2 was at price parity in the US vs. rest of the world, it would be a reasonable value IMO (and would make the X2 Pro kind of irrelevant IMO). It's not clear that the Polar watches are better than similarly priced Garmin watches IMO. Looking for price parity on similar (maybe worse?) watches vs. the brand with the highest name recognition / snob appeal in the market doesn't seem like a good strategy for the much less well known small brand to me. Brand wise, Polar is probably more on par with Suunto, but the value is so much less with Polar. Vertial 2 is $600, and probably comparable to the X2 (pro?). Race 2 is $500. I would strongly prefer to buy another Grit series watch when my current watch dies. And I would also prefer to stay in the Polar eco system (unlike many I like the app vs. hate it...and I also think that staying with Polar will mean fewer connectivity issues with my H10s). But I can't justify spending an extra $300 - $500 for a comparable watch when compared to Suunto. And if I was going to spend that much, I'd probably just get the Forerunner 970 and save the $50 over the X2.
FR965, bought in 2024 and ditched my Polar Vantage 2. Because i didn't owned a Garmin before i cannot say which features are more useful for you. From my point of view i totally enjoy features for running splits on to have a more structured training in pools.
End of reviews
Garmin
Forerunner 255 Series
Long battery; but wrist HR tracking is inconsistent.

Garmin
Forerunner 955 Series
Best value, clear MIP screen; limited smartwatch features.

Garmin
Fenix 7 Series
Durable, multi-week battery, flashlight; high price, buggy software.
Garmin
Instinct 2 Series
Rugged, solar multi-week battery; bulky, small screen readability issues.

Garmin
Enduro 3
Multi-week solar battery, lightweight; but bulky form factor.

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Oura - Oura Ring Series

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Apple - Ultra Series

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Garmin - Enduro 3

Ranked #1
Garmin - Fenix 7 Series

Ranked #1
Garmin - Forerunner 255 Series

Ranked #1
Oura - Oura Ring Series