Recommended DNS server for a medium 20-30k users isp

[DurgaPrasad - DatasoftComnet via NANOG <nanog () lists nanog org>]

Hello all,
Do you have any recommendations for recursive DNS servers for a medium sized (20-30k users) ISP.
We have used powerdns and unbound but sometimes find the caching times a bit on upper side. Any suggestions between 
these two or anything new?
Also need points on how much we tune the settings
pros and cons if any.

Thank you /DP
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