In terms of speed, both offer about the same throughput.
OpenVPN is open-source, which means anyone can inspect its code. It's generally considered the safest option for that reason. IKEv2 isn't known to suffer from any security issues, but it's not open source.
If you're on the move and using a VPN on a mobile device that routinely switches networks or loses service, IKEv2 is preferable. It reconnects much more quickly.