The bottom line is what functions are you using with your clients. What functions do your clients use independently of when they are with you?
New clients often do not need detailed functions, but just the basics. Heart rate monitors must be an effective tool for the client, and not what the trainer thinks is best. In other words we may like the multiple functions because we use them, our clients may not.
Polar is excellent, but my old model does not allow me to change my own battery locally, I must send it in. Other models are less expensive (Omron $35) and may fit client needs.
I agree with Michael. I have always liked Polar. They are accurate and dependable. As Thea suggests, the FT-80 is a nice monitor.
As Susan suggests, finding a heart rate coindent with the onset of hyperventilation (lactate threshold) is a great idea. I like using a treadmill of stationary cycle to determine the HR/threshold Then, do intervals up to that HR, slow down 25-30 bpm. Repeat the intervals for whatever time you work with your client.