Hello,
The biggest difference between insert vs dual mode for a pre/post setup is the ease of use and setup. Dual Mode has “pre/post” mode and is much simpler to setup and use, this would be my recommendation: https://cdn.eventideaudio.com/manuals/h90/1.10.7/content/setup/pre-post.html
The H90 introduces a very small amount of latency that most users do not perceive. I’d suggest doing some basic playing tests yourself before deciding on how you want to setup your rig based off of latency figures.
There are latency specs in the manual here: https://cdn.eventideaudio.com/manuals/h90/1.9.7/content/appendix/specifications.html
Your preset routing and if you are using external inserts or not will be a bigger factor in the total latency compared to insert vs dual mode.
Let me know if you have any further questions.