I want to use the iim7-5. What's the difference between using it from C Natural minor and C harmonic minor?
You've realized yourself that there can be several options for scales from which a chord is borrowed. A IIm7(b5) chord in a major key is usually considered to be borrowed from the parallel minor key, and not from a particular minor scale. So it's your choice which of the possible minor scales (natural or harmonic) you would use to improvise over that chord. There is no correct or wrong choice, it's a matter of taste and interpretation.
Note that you might as well use the harmonic major scale over that chord. It is actually a very natural choice because it only changes one note from the major scale (6 -> b6) to make the IIm7(b5) chord fit in. The minor scales also change the third of the major scale (natural minor as well as harmonic), and the seventh (natural minor), even though these notes are not part of the IIm7(b5) chord.
The minor scales all start with the same five notes - in Cm, they're C D Eb F G. Then there are changes. C harmonic has Ab and B, but C natural has Ab and Bb.
The iim7b5 will have D F Ab and C, as it starts on D, the m3 is F, 7 is C and b5 is Ab. So, the notes that will work better are from C natural minor, with the Ab. C harmonic with its B will be fine - until you play the Bb note - when it'll clash.
There is no difference in the ii7-5 chord between C natural or C harmonic minor, since both contain an Ab. The only difference would come in the C melodic minor, which would raise the Ab to an A-natural. In that case, there would be no difference between that chord and a regular iim7 in C major. That being said, it does not really matter where you are borrowing a chord from if it works. My composition professor gave me very good advice and told me not to try to analyze my own music.