Updated question to this. What's the effective difference between the two models?
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I can't tell you any more than PS Audio's web site does, other than that the 20A model does indeed work better with a power amp. But I CAN tell you that if you have one of the original UOs without the fuse light, you should have it updated. I actually hardwire bypassed the fuse (NOT recommended if you aren't willing to accept the potential consequences!).
Read the Stereophile Review in their Archives at www.stereophile.com . They cover both versions in their review.
While I would go with the 20 amp version over the 15 amp, get the 15 amp IEC with it instead of the 20 amp. PS Audio tells me the 15 amp outlet is capable of about 40 amps, so will allow the 20 amp Ultimate Outlet to fully function, yet be far more compatible with the majority of power cords.