What are the practical differences between glutaraldehyde and formaldehyde for fixation?

I’ve recently been trying to understand the practical differences between glutaraldehyde and formaldehyde when used as fixatives in biological sample preparation.

Both are aldehydes and both cross-link proteins, but I’ve noticed that different sources describe their behavior in slightly different ways. Some say formaldehyde fixes tissues faster, while others say glutaraldehyde provides stronger fixation and better structural preservation.

From what I understand so far:

Formaldehyde seems to penetrate tissue faster

Glutaraldehyde appears to create stronger cross-links

But I’m not completely clear on how that translates into real laboratory use.

I work with a chemical supply company (Hoochemtec) that provides aldehyde reagents to labs, and occasionally we get questions from customers about which fixative they should choose. I’d like to understand the reasoning from researchers who actually use these reagents in microscopy or histology workflows.

A few specific questions:

Why is glutaraldehyde preferred for electron microscopy, while formaldehyde is more common in histology or immunostaining?

Is the main difference related to molecular size and tissue penetration, or to the cross-linking chemistry itself?

When labs use paraformaldehyde + glutaraldehyde mixtures, what advantage does that combination provide?

In practice, how do researchers decide which fixative to use for a particular experiment?

I’d really appreciate hearing from people with experience in microscopy, pathology, or lab sample preparation.