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.
It'd be a recovery ride Id be doing if my heart rate was high. A Pwertap is a good way of telling you if you're not recoverd as well cos if you're heart rate is too high at your target power it's a good sign you need more rest. Resting heart rate has been too vague in my experience. Too many factors can affect it. But thats following a strict program. If it's just to get a bit fitter then a HRM would be fineI should think. People managed with only them for years