IPv6 is whether more secure nor less secure than IPv4. Both versions are secured with IPsec.
The difference is, that IPsec is a mandatory part of every IPv6 stack, whereas with IPv4 you have to install IPsec separately.
In the beginning IPv6 may be less secure, because the stacks are immature. Take into account that we still find security holes in IPv4, so even more with IPv6 being a “young” protocol, there will be more immaturity features and bugs in the beginning.
Which means we have to test everything carefully and make sure our firewalls know how to do their IPv6 job.
Hope this helps