I have used a powertap in the past and it was pretty good,only problem is they are very prone to breaking(mine is back in the States for the 3rd time)an SRM is the best but these are BIG bucks and need regular calibration.Also to get the best from a power meter you really need expert advice/analysis as while all the information and graphs look pretty most people can't make head nor tail of them,including a fair number of coaches.For a cost/performance HRMs are hard to beat and as for your heartrate being 20 beats lower for a certain wattage from training,you should already know you're tired from taking you're resting HR every morning and previous experiance of hard sessions and back off accordingly,none of these devices are any good if you can't listen to what your body is telling you.