You can use both, but I've never found a good reason to at home. The DHCPv6 option for specifying an NTP server might be a good reason to... but even Linux machines ignored it when I tried. Maybe in five years :)
work to get my Linux systems an IPv6 DNS and NTP server (respectively) when I'm using ISC DHCPd and 'dhcpcd'. Perhaps your DHCPd and/or your DHCPc were either misconfigured or badly broken?
It is a little weird to be using the 'sntp' option, but on the systems I have access to, the address passed back to the client is treated exactly the same as the DHCPv4 NTP information. ~Thirteen years ago, one of the ISC DHCP bigwigs seemed to be of the opinion that the DHCPv6 NTP Server Option was "badly broken" [0]. Perhaps that's why the 'sntp' option is used instead.
