A few weeks ago I was looking for the same thing. The only metric I'm concerned with is Cadence.

I ended up with a Wahoo Cadence sensor. Here's the best part: It's wireless and I wear it on my shoe.This way, it's no problem to jump between bikes. I bought it with the hope that I'd somehow be able to monitor cadence using an Apple Watch through an app. We'll it might be a little premature for that use, but here's how I do use it...

It synchs with the Wahoo App on my phone. I took it for a test ride with the phone and it worked well but I have no interest in mounting my phone on my bike(s).

Enter the solution: The Wahoo Reflekt computer. The phone is still the brains of the operation but you can keep it tucked away in your bag. The Reflekt is just a remote screen for the app. The phone "throws" the data to the the Reflekt computer on the headset.

Best part... You can choose from a plethroa of premade screen configurations or make your own by displaying any number of metrics. I do cadence and time.

If you do go this route I'd buy the two (Cadence, Reflekt) as a package. I paid an extra $20 because I poossay-footed into it, buying one unit at a time.

*** The forum blocked the correct spelling of poossay-footed, though it literally means when a cat walks gingerly.***