A Watt is a Watt - Power Training vs Heart Rate Training

Your HR zones don't change much except very slowly as you get older.  One of the nice things about a power meter is that you can see exactly how strong you are riding.  If you keep good records you can see your progress or regression over a long timespan.

There are other things you can do with a power meter, like calculating your CdA to improve your aerodynamic position on the bike.  Pick up one or both of these books:

Training and Racing With a Power Meter

The Triathlete's Guide to Training With Power

If you're not really digesting what's in those, or just not interested, then I wouldn't make the investment in power.  It's just an expensive bike computer if you're not taking advantage of it properly.

Most important : you can brag about your 370watts FTP but can't with your 192 LTHR

LOL...maybe you can brag about 370 here on BT, but you'll get laughed at with anything less than 400 on ST. 

In general, you can see improvements by comparing pace at the same HR.  But you are making the assumptions that the outside variables like wind, hills, temperature, etc are constant. 

This is the point where you need to break that barrier that a lot of people use their gadgets as laws. I've advocated even racing naked (gadget wise) several times based on this mindset: If you've put in enough training hours for your particular race, you become very familiar with how your body feels and reacts to different effort levels. You'll know if you're pushing too hard because your muscles just don't feel like they should. They're great tools, but outside factors can even impose on the effectiveness of using a power meter. You know your body and how hard you should be able to push. When you use a HRM or power meter for a long enough time you can also develop a sense of what numbers you should be hitting based on how you feel. So use them as guidelines rather than limits because if heat is causing your heartrate to beat 8 times per minute faster than normal, but your muscles are still working at the same effort level that they normally do, then race at +8 BPM. Drink more water, make sure you stay hydrated, and get in enough electrolytes to sustain that effort level in the heat. If you're sick you're going to be reading power numbers smaller than other race conditions, and adjust your sustained power to be a little lower than normal.

I was primarily training with HR for the past few years and HR is good stuff. Def better than no data in terms of getting pacing and zones roughly dialed in.

However, I recently acquired a Powermeter, and while it's not the be all end all of training, it has revealed significant weaknesses in HR based training, similarly to the scale when I compared HR to no HR training.

I've been doing the same 56 mile solo ride with 5500 ft of climbing every Sat for the past 6 weeks, so I have a very good repeatable gauge of fitness with this ride.

Here's the breakdown:

- My average time (for what it's worth) on the ride has dropped by 20 minutes. I try and go close to all-out on these rides, so it's not just a matter of effort that's making me go faster. I FEEL notably stronger on the rides lately (much moreso) so it's also a real fitness effect.

- Normalized Power on these rides has gone up 20 watts over the past 6 weeks. It has gone up 3-4 watts on every ride, reliably as I've been ramping up training. This is good OBJECTIVE proof of improving performance.

- My HR results on these rides though, first went up as I could last longer at harder efforts, but then have started going DOWN. I was tracking HR using a weighted HR score called "TRIMP" in Golden Cheetah that's very similar to TSS in that high HR efforts get more weight than easy ones in the final score. Ends up really hard to interpret the HR data for improvement since the stronger you get, the harder you can go, but also the lower your HR for comparable efforts. 

THe HR works better on steady state long interval workouts, but I'm finding that %time in zone is very hard to interpret as an objective measure comparing workouts given that when you're improving, your ability at each zone changes even on a week to week basis.

While I agree with your assessment to a great extent, after 3 years with the power and pace, I can tell you that you need both, HR and power in order to correlate INPUT/OUTPUT. If anybody tells you it is one or the other, it is grossly omitting valuable info. You need to know the relation between the two at any given moment. For tracking long term improvement you also need to observe rate of decoupling of the two under different loads. Very telling.

Powermeter did not improve my cycling at all. I knew how to train well before it, with only HR and RPE. It did improve my racing as it opened up my eyes on pacing the bike for a solid run. That is the only benefit I have seen from powermeter.

I can't hold the same in either metric in those situations.

Right.  The point I was trying to get at was variables that affect pace.  Since it was said you can see improvements by comparing pace against a constant HR.  I probably should have left temperature out above.

If rip downwind for 10 minutes at 160 bpm at a 30 mph average, compared to last week where I was riding with a crosswind for 10 minutes at 160 bpm at a 22 mph average...how do you compare these two rides? 

With a power meter...simply look at the watts. 

Whoever said earlier that you have to take HR AND Power into account is exactly correct. Each on their own have merits and limitations but to truely get the entire picture of an athletes performance you need to look at them together. Sometimes I will have athletes ride by HR and sometimes by Power, sometimes we'll cover up both and ride by "feel"/RPE and compare the rides. Almost everytime the Power ride was the most consistent, the RPE/feel ride was the worst (pace and performance) and the HR ride was in the middle. Interval training on the trainer was the same, in a series of 5x5 min intervals the athletes tested had the smallest amount of deviation from the first to last interval using Power as the primary metric, using HR the 5th interval was as much as 10-15 watts lower at the same HR, and the feel/RPE intervals were just plain awful by the 5th one (as much as 30 watts lower).

Having said all of that about power, you have to be in tune with how HR relates to certain power outputs in certain conditions and know when to back off on wattage.

Keep in mind (and this is paraphrasing Allen/Coggin) "Heart Rate tells you how hard your heart is pumping, not how much you are improving on the bike. Power tells you exactly how hard you are pushing on the pedals."