cyclezen

in Direct response to OP's initial Q - I would offer a Heart Monitor (HM or HRM) as an option (or to be certain the HM component is also part of the 'power' system).
why?
it is also a direct link to your body performance. A PM really needs an HM to fully qualify the info you're getting. A PM will tell you your output at the bike interface, but without a HM component it's not telling you at 'what expense' ... Combined the 2 will give you an understanding of your 'efficiency' at whatever level you're riding.
now the side comment...
I have WAY Tooo many bikes to consider having a PM on even a small selection. I do try to ride them all, but have 4 which are the most 'often' ridden. I can't imagine setting up 4 bike PM systems... crazy !
If I had only 1 or maybe even 2 bikes - possibly...
I started training using a bike based HM in 1986 - make a big difference in my training for racing... But I was already a few years into "veterans" cat (but still racing 1-2-3) and with much less time available. The HM really helped in maximizing my bike time.
I always wear it (the recent one), on every ride, every bike, and sometimes on hikes/backpacks or ski adventures.
You can really get to know yourself, once you have some period of 'data' about yourself.
The modern HM/HRM can give quite extensive data, like KCAL, if 'tuneable'. From that you can calculate some general 'power' numbers which give an approximation.
Enough for me.
I tried the wrist -based system - FitBit - and found them wildly inaccurate, hard to read, and just not usable. I had hoped they would be a good replacement for the chest strap systems, especially when it came to XC skiing or hiking/backpacking. ... NOT....
One thing the HM does give me quickly, is my general 'state' at the beginning of any ride, as relates to my 'state' on that day. A fast spiking of HR or low 'ceiling' rate at perceived higher effort tells me my body/system is just not close to an optimal level... Helps me decide when I need to think 'recovery' as opposed to 'hammer' kinda day.
Still do the ride - but with a more reliable focus. Given I stopped racing in '13, I can now choose how any day will shape up, and not driven to 'max' each ride - A bad habit I've always had... LOL!
I Still have 'goals' and 'objectives' and hoping to again ride near the 'front' - but thankfully they're tempered... and my riding buddies like me better this way also... LOL!
Ride On
Yuri

Carbonfiberboy 

OTOH, on really long rides, even though I have a PM, I pace by HR. I find the guys who pace by power don't realize how tired they are getting until it all goes Plunk. HR keeps me better informed of my physical situation, which causes me to adjust my power to my current ability. I still use power to stay smooth, but vary the power according to my needs. My usual tactic is to set a HR limit for a long ride and not go over it - not a power limit.

PeteHski

I do that too. They are both useful and best used together. I do quite a few long, hard events and still see a lot of people blowing up in the last hour, especially if there are hard climbs near the finish. Using both power and HR pretty much avoids that scenario for me.

Flip Flop Rider

if you have the money and that's not an issue, why not treat yourself