Power tap is not something I know about, GrasB uses them, as does Vamp, however, as a general comment you could save money by using an existing Garmin head unit instead of the Cyclops one, same re. HR strap. I have heard the lifespan on the torque tube on Powertap wheels is fairly limited and costs a lot to have replaced, also heard about issues with them going out of true a lot. But this is just hearsay.
I use a Quarq. For me, a wheel based PM is no good, because I want to train and race with power and can not afford 2-3 different wheel setups (training wheel, disc wheel and lightweight wheel), I can swap my Quarq from one bike to another in less than 5 minutes. If you wanted to consider crank based power measurement, at powertap prices, you may want to consider power2max. There is also the Quarq Riken at £1250-1300 depending on bottom bracket type.
Whatever you decide, I would recommend speaking to
Cyclepowermeters and could recommend them as a retailer. They are brilliant! The kind of guys you can ring up and hash out a problem over the phone and get advice on the spot. If something fails, they offer a very fast turnaround. Their staff team really know their stuff, some of them being active coaches (good ones!) and also very good racers and furthermore, they are active on various forums, i.e. they are actively involved in the use of the equipment in various capacities, rather than just sales people.
Re. training with power I benefited massively, there is no way I would be riding 21 minute 10's at this point (based on my starting point) without having trained with a power meter this season! Not because I didn't train hard and consistently, but because I clearly didn't train hard enough on hard days and easy enough on easy days. I found it hard to quantify my training load and correlate it to how well I was going etc. Power changed all that. In races, power is a mixed bag for me, I have found on occasion, it helped my pace well, letting me ride to a pre-determined schedule, on others it messed with my head. My PB's have come when I relied on power for the 1st mile or so (to avoid over cooking it) then racing on feel (90% anyway, I still sneaked a peak at the numbers now and then, mostly to avoid blowing up). My data analysis, especially when reviewing road and race data is relativelly poor (for my indoor training, it is good as it needs to be), a club mate reviewed some race data for me and pointed out a few things I had completely missed. This shows that to get the most from it, you need to know what you are looking at/for.
The main issue I see in recommending power as a training metric is that how much one person benefits compared with another will vary quite wildly, you need to at least have a rudimentary understanding of the concepts and what it is you are looking at else it will just be a load of numbers that mean nothing. As Mr Haematocrit said, you do need to be structured with your training if using power, this just doesn't suit some people personality wise. Same goes for analysing data, some people can't be bothered or don't like doing it (could always hire a coach to do it for you though). It is also quite easy to get obsessed with the numbers and forget the bigger picture.
In short, the short term improvement vs say, some bling wheels or whatever is not favourable. But the long term gains from training with power usurp the benefit of a new bike or wheels, IME.