I saw Frixion pens at lqs but haven't tried them.

All I can tell you is, don't use the orange chalk marker that comes in the variety pack for Clover chalk holder pen. Ask me how I know.

I have been using

Do you iron away the marks of frixion pens? or is it safer to blow dry the marks away? Are these pens expensive? I will give them a try.
Our quilt guild is doing a fabric swap using scrappy half square triangles. I made pinwheel blocks and each section of the pinwheel is a different fabric. I will trace a circle template over the scrappy pinwheel block to cut out a circle. The challenge is to find a marking pen that works on 8 different fabrics. ha!

I use the sewline pencils and have never had a problem. Different colored lead for different fabrics values.

From experience I go the cheapest and easiest, charcoal pencil. Press lightly is all you need and it comes out clean in the wash. No troubles so far! Good Luck whichever way you choose.

I iron the frixon marks off. I wash the item and all the pen residue comes out. If you don't wash, the marks will reappear if below 32 degrees.

I use them for making diapers all the time without issue

I use the washable markers at times when my chalks and washable marking pen aren't doing the job. I'm careful to choose a close color to whatever I'm marking, not significantly darker. I haven't had any problems so far.

I use them all the time, the only thing I don't like is if you try to just spray out the marking it makes a mess but when you wash it all the markings come completely out.