Trigram or arrow in navigation menu for e-commerce?

Any case studies/best practices that can help me decide which to use? Need to convince the client, but also choose the best for users.

The plan is to have 1-2 expandable categories sitting far left or on the left, with no more than 5 categories that don't expand.

I'm a huge fan of the trigram and think it's the future, but maybe it's still confusing for consumers? Need to let them know which categories expand as clearly as possible. Visitors mix of ages, but most 25+, many 55+

Happy to hear your thoughts and experiences.