Having a chronograph for a hunting rifle is not really necessary, but is insightful.
Having a chronograph to determine how well and consistent your ammo is, it is absolutely beneficial (ES, SD, long string recordings for consistency testing, etc.). Just depends on how far down the "accuracy/consistency" rabbit hole you want to go.
Of the 3 mentioned, I'd say the Garmin gets the top recommendation.
The Magnetospeed is just a PITA to use, and wastes ammo, since it will most likely change your POI, requiring additional ammunition to verify zero (in addition to the ammo used to verify speed/consistency).
As to Athlon and Garmin (both good units), Garmin gets the nod. Simply because Garmin have been supporting small computerized devices for 30+ years now (remember all those Garmin mapping devices?). They literally wrote the book on how to manage, support and update a wide variety of devices with varying states of firmware ages (skipped the last 2 updates? No problem, Garmin understands which firmware version has what dependencies on previous firmware versions that you may have missed while the device was stored away). And nothing is more frustrating than buying a nice bit of kit, but having to battle with updating it.
Like I mentioned at the beginning; do you need one for hunting ammo? No. But it sure helps you understand the quality of your ammo, as variables change (new lot of powder, new batch of bullets, different brand primer); only a chrono will tell you the minute differences.
In addition, most UK reloaders don't load beyond more than 100rds at a time (be it component restriction limits, or practicality of how much ammo is used in a given year). But given that lack of volume, a chrono will help you narrow things down, between batches of ammo loaded. It most likely won't matter on a deer at 100yds, but it will as you learn how to make the most accurate ammo you can.
JMTCW....