Hello Muhammad,
It does no matter, it depends on your requirements but good practise is to not mix up both types  in one domain. Private IPs are not routed in internet, so if you run BGP you have to use public IPs.
But if you run MP-BGP to support MPLS VPN in your network, you can use private addresses.
It really depends on scenario to decide if to use public or private IPs.
Best Regards 
 
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	Best Regards
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