I just made this comparison between Withings Body Cardio & Renpho Smart Body Fat Scale on my blog:
Is there anything else that should be included? Maybe I only have a few days more for testing 
The following is an excerpt from my blog post:
…
Accuracy of the scale
Bioelectrical impedance scales are not accurate, and should mostly be used to track changes over time.
Weight:
Their weight measurements are ALMOST the same!
Muscle mass:
Muscle mass measurements show almost the EXACT same trend.
Fat mass:
For some reason the fat mass measurements don’t match up…
This is the fat mass from Withings compared with the visceral fat measurements from Renpho (the blue line at the bottom).
This is the fat mass from Withings compared with the subcutaneous fat measurements from Renpho. They also don’t seem to agree…
Winner: Not sure…
They show the same trends in weight and muscle mass, but differ in fat mass. The problem is that I don’t know which one is more correct…
…
How can I figure out which one is the most accurate? (their weight & muscle mass measurements are similar, but not their fat mass measurements) (Without expensive DEXA scans)