I need to measure the impedance of the system, is it better to measure the resistance or capacitance?
What you need to measure depends on what you are doing and what is most useful for that task.
For example, this paper deals with Retinol detection.  It mentions the &quot;Single Frequency Impedance technique.&quot;  If you were trying to get results comparable to those in the paper, then you would have to implement that technique - nothing we tell you about how to measure either resistance or impedance matters unless it applies to that technique.

&quot;Resistance&quot; is a purely real impedance.  You can (normally) treat a regular resistor as a purely real impedance.  At high frequencies (or with particular construction methods such as wire wound resistors,) resistors are less than purely real - a resistor at extremely high frequencies acts like a combination of a resistor, a capacitor, and an inductor.
Capacitors and inductors have an impedance that is not purely real.  The complex values for the impedance have a frequency dependent effect on the signal passing through.
You can't say you want to measure the impedance of some unknown thing, then equate that with measuring the resistance, the capacitance, or the inductance.
You must measure the impedance.  The resistance, the capacitance, and the inductance of what ever you are testing (together with the same properties of your test setup and equipment) will combine to form an impedance - and it will quite likely be frequency dependant.

You need to look into existing methods for measuring Retinol levels using impedance.  See how they work. From there you can decide whether to implement a standard method or try some variation (or something new.)