Some criteria you might want to consider:
Do I plan to switch out wheels sometimes? Then G3 is not what you want.
Do I want double sided power?
Do I want L/R power balance? If so, do I want estimated or actual balance?
Do I want Torque Efficiency and Pedal Smoothness?
Do I care about the looks? (since there are many good quality power meters, might as well choose one that looks good too, for the given price range)
Do I plan to disassemble and wash the chainrings? I think C1 can be separated from the crank and washed but you would have to go through the break in again, AFAIK.

I personally got a Powertap C1 on sale for $524.99 no tax and free shipping. I like it a lot, especially for the price. Very good value dual sided estimated L/R balance power meter.

Here's a little chart I put together from DC Rainmaker's 2015 Power Meter Buyer's guide. The comments, especially the negative ones are based on my criteria and opinion so don't be offended if you have own one of those I don't like.


[TABLE="width: 577"]
[TR]
[TD]PM (no particular order)[/TD]
[TD]Placement[/TD]
[TD]Comment[/TD]
[/TR]
[TR]
[TD]1. Powertap G3[/TD]
[TD]Rear Wheel[/TD]
[TD]don't like the hub sound[/TD]
[/TR]
[TR]
[TD]2. Quarq/SRAM[/TD]
[TD]Crank Spider[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]3. Power2Max[/TD]
[TD]Crank Spider[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]4. SRM[/TD]
[TD]Crank Spider[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]5. Powertap C1[/TD]
[TD]Crank Spider[/TD]
[TD]can't wash chainrings easily; looks good[/TD]
[/TR]
[TR]
[TD]6. Rotor[/TD]
[TD]Crank Arms[/TD]
[TD]DC does not recommend[/TD]
[/TR]
[TR]
[TD]7. Stages[/TD]
[TD]Crank Arms[/TD]
[TD]left only[/TD]
[/TR]
[TR]
[TD]8. Pioneer[/TD]
[TD]Crank Arms[/TD]
[TD]ugly red pod + crank arm stuff[/TD]
[/TR]
[TR]
[TD]9. 4iiii[/TD]
[TD]Crank Arms[/TD]
[TD]unproven[/TD]
[/TR]
[TR]
[TD]10. WatTeam[/TD]
[TD]Crank Arms[/TD]
[TD]sucks[/TD]
[/TR]
[TR]
[TD]11. Garmin Vector[/TD]
[TD]Pedals[/TD]
[TD]sucks[/TD]
[/TR]
[TR]
[TD]12. Powertap P1[/TD]
[TD]Pedals[/TD]
[TD]Look only[/TD]
[/TR]
[TR]
[TD]13. Polar/Look combo[/TD]
[TD]Pedals[/TD]
[TD]Look only[/TD]
[/TR]
[TR]
[TD]14. bePro Look only[/TD]
[TD]Pedals[/TD]
[TD]Look only[/TD]
[/TR]
[TR]
[TD]15. Xpedo[/TD]
[TD]Pedals[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]16. Brim Brothers[/TD]
[TD]Cleats[/TD]
[TD]not out yet[/TD]
[/TR]
[TR]
[TD]17. Ashton Instrument[/TD]
[TD]Bottom Bracket[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]18. Dyno Velo[/TD]
[TD]Bottom Bracket[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]19. Rotor INpower[/TD]
[TD]Bottom Bracket[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]20. PowerPod[/TD]
[TD]Non-direct force[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]21. LEO[/TD]
[TD]Non-direct force[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]22. iBike[/TD]
[TD]Non-direct force[/TD]
[TD][/TD]
[/TR]
[/TABLE]

+1

It works for me well.

Not sure what Rchung was trying to say, though, with linear regressions and stuff. Im-press-ive!