You can use both although IBRION =5 is time consuming and IBRION =6 is less time consuming as in IBRION =5 atom is displaced in all the  three coordinate while symmetry consideration is taken care in IBRION=6. FYI, please write the INCAR file carefully as once the IBRION=5 or 6 calculation for vibrational frequency fails or stops in between then you have to do the calculation again, there is no other way around to start the calculation from the point where it finished.

Hope this helps.

Yes, That's what happened with me. wall time for the supercomputer expired after 72 hours and calculation stopped in between. Which other methods for frequency calculation will you suggest? I tried installing Phonopy but I did not get how to use it.

Thanks

I have used phonopy if you have any specific question I can tell you.

i have installed it, but now what are the stepwise usage of it as i saw some tutorials which are not helping, like how to make supercell, how to use it with VASP. I will say i know zero about how to use it. If you can suggest me I will be so grateful. Also Can we communicate through email if you do not mind?

Thanks 

Absolutely IBRION = 5 can be used to calculate vibrational frequencies as it was guided in VASP manual (https://cms.mpi.univie.ac.at/vasp/vasp/IBRION_5_IBRION_6.html). However usually you need to use higher convergence criterion. e.g. EDIFF = 1E-6 and lower POTIM, e.g POTIM = 0.015 to get the results more accurate.