I wouldnt buy blast because they require you to sign up a yearly fee to see all the data.
I have a zepp gen 1. I think it isn't as accurate as blast and dk (and probably zepp gen2) and reads high on batspeed. It is a nice tool but I wouldn't use it every time, maybe once a week to check your metrics and then create some drills in case something is off. When you actively work on something you can use it more of course but you also want them to develope feel for the barrel and then cross check against the actual data and not just rely on getting a read every swing.
Most important metrics are attack angle, vba and bat speed but the sensors also have compound scores now. I prefer the raw data tough.