I just got my Karoo 2 on Friday and took it out on Saturday. Coming from a Bryton, it's a huge difference. If you've got an Android phone, it functions very similarly and is quite easy to set up.

Polar.

but that's just me.

why do I like Polar? because i started with it over 25 years ago. my current Polar is the vantage M (i think) I still use my old polar rcps3 (I think) on one of my road bikes. I can measure all the vital stuff. but most importantly it measures cadence. I am too cheap to go blue tooth on that bike so I strap the watch on my handlebars ... it works well..

ON my other road bike ( A giant defy) I will eventually get a bluetooth thing. I would love to see Polar go bluetooth. anyway....

buy what you think will fit your needs over the next 10 years. Garmin and I think Polar will be around for a long time.

Buy a monitor that will keep you interested on the long lonely rides.... a certain percentage of enjoyment is documenting your statistics.... i know I like doing that.....

have fun and don't over think this ..... lots of companies provide the same services. They just do it with different graphics.