Technitium is a full fledged authoritative recursive DNS resolver with adblock, whereas pihole is more focused solely on the adblock experience. Technitium supports full zone management, DNSSEC, and all DNS record types. Pihole is limited to a handful of types and can't do custom zones and the management is somewhat clunky IMO. Technitium has full DNS logging, statistics, conditional forwarding, and a full rest API for management.
I think Pihole is a great project, but Technitium caters better to power users and people that want/need more complex control over their internal DNS infrastructure (while still remaining relatively simple from a management perspective).
Simply put, Pihole is adblock with some DNS sprinkled in, Technitium is a DNS server first with adblock support. I don't think you can go wrong either way, but if you're going to need more advanced DNS capabilities at home, roll Technitium to save yourself a migration down the line.