Since I wasn't convinced training with power was all it's cracked up to be, I went the cheap route and bought a computer that is calibrated to estimate power when used with my Kurt Kinetic trainer. Not the most accurate tool, I know but it was less than 1/5 the cost of some other power meters. I've been using the trainer and power meter over the last couple of months and I hate to admit it but it does work much better on shorter interval workouts than heart rate data.
Heart rate takes a while to creep up when you are doing a hard interval. So it doesn't give you good feedback on your effort if you are doing shorter intervals, like 1 or even 2 min. You get immediate feedback on how hard you are pushing with the power meter. That's key when you're trying to improve your high end output. Since this is my primary limiter... I guess I should stick with it.
The down side: It really effing hurts to push that hard and I highly recommend you keep a bucket close by. I guess I should also add that it takes some homework to find out what your ideal wattage for training a given interval length should be. That's something that a lot of data hounds out there will embrace. For the rest of us, it's a hassle.
You can download a pdf file at www.trainingbible.com that thoroughly explains training with power called, you guessed it... Training With Power.
The first person to puke wins.