GLM is more flexible, so use that

Dr. Andrew is correct. You should go for GLM.

Thank you so much for your advice.

The linear (mixed) model is just a special case of the generalized linear (mixed) model, and the choice should depend on the nature of your data. For count data, use GLM(M) from the Poisson family, for binary data a logistic regression model, for continuous data you may use a linear model.

I'm not sure which software package you're using, but maybe this overview is helpful:

http://htmlpreview.github.io/?https://github.com/strengejacke/mixed-models-snippets/blob/master/overview_modelling_packages.html