Forerunner 970 and hrm 600

Hi. 

I’ve just purchased my first Garmin watch and I upgraded the hrm strap from pro plus to the hrm 600. 
I wonder what connection I should use, ant+ or Bluetooth secure? 
I plan to the Core 2 heat sensor to it with ant+. 
And maybe an Apple Watch Ultra 2 as well, but that’s not necessary if it’s complicate things. 
I have a polar arm strap I can connect the Apple Watch to. 

thanks in advance for the replies.