220-your age is not very accurate,you need to do a max hr test(although you should see a doctor first).My max hr is 201 but 220-age would give 188,this would throw all of my zones out and make the use of a HRM almost pointless.Hope this helps.
My heart rate can be as much as 20 bpm out at a given power depending on the training Ive done the previous day, so not the best way to train at all
Not sure any of these are ideal - whats needed is an appreciation of the pros/cons of HR, Power, and how you feel - ie if you feel shit one day, back off!
Regarding the 220-age max HR - this has been known to be very approx for years. I knew there were other approximations around, quick wiki gives..
For me, tested max HR by Sally/Sally methods on Turbo gives 202bpm
220-age = 187
HRmax = 205.8 − (0.685 × age) = 183 (SD of 6.4!!)
HRmax = 206.3 − (0.711 × age) = 183
HRmax = 217 − (0.85 × age) = 189
Which are all pretty way out.
The other important factor when training is to identify your anearobic threshold - this is a good measure of fitness and useful for training.
I am sure a power tap has its uses, but for most, just the basics of a HR monitor and understanding of training to suit your own needs/targets will give a noticeable advantage IMO.
Have known a few guys use powertaps, and although useful, comments were more that it was useful for getting max power out for TT type events.
If you are already near peak fitness and looking to gain theabsolute max on a structured life training program, sure there is some use.
I still find best thing is just go out and ride with people quicker than you