As a little background, I own (and use) the Hydra, the P300, and the P600. However, which unit is "better" depends on the particular application and the quality of the power in your home. For example, using the P600 with CJ Eight XS's produced a whitening of harmonic textures, a reduction in soundstage depth, and most annoyingly a muting of dynamics and transient attack. Thus, if you plan to use the power conditioning for larger power amplifiers, I would prefer the Hydra. However, for both digital equipment and turntable power supplies, I find the Power Plants win hands down. For digital front ends, the Power Plants produce a significant reduction in the noise floor (which the Hydra did not), improved bass definition, and a more transparent window to the recording. As a caveat, though, I live in a condominium complex with terrible power (voltage ranges from 95V to 120V, all kinds of garbage on the line). As a second caveat, some users have indicated on AudioAsylum that they experienced transformer noise problems with the Powerplants. I have (fortunately) not experienced any such problems.
Thus, without knowing more about your system, I think the best thing you can do is see if you can arrange a home demo of both the Hydra and a Power Plant. I believe several dealers carry both units, and would possibly be willing to arrange an in-home demo (if you definitely plan on buying one or the other).
On a side note, I also tried a Richard Gray Power Company 400 in my system, and was not impressed - either the P300 or the Hydra would be a better choice.
Thus, without knowing more about your system, I think the best thing you can do is see if you can arrange a home demo of both the Hydra and a Power Plant. I believe several dealers carry both units, and would possibly be willing to arrange an in-home demo (if you definitely plan on buying one or the other).
On a side note, I also tried a Richard Gray Power Company 400 in my system, and was not impressed - either the P300 or the Hydra would be a better choice.