Several of the conversations I've had with teachers or heard teachers having with students have been about paper choices writers make. Everyone seems to agree that writers should have choices depending on the purpose and form of their writing. Within those choices, there are also more specific versions for different kids. For example, students might choose paper with space for an illustration and lines for words, but then there might also be different sized lines to account for fine motor skills.
For some reason, I haven't heard these conversations about writing utensils. Students are generally given one size pencil or a pen. Thinking about your students, it probably makes sense that some would be more comfortable with fat pencils, some with skinny, some with short and some might need a grip. Pens are the same- for some, they have as much control as they would with a pencil while for others the amount of extra effort it might take to control the pen takes away from the writing.
Take some time to observe your students during writing and think about the choices they are making and the ones you are making. If you notice a student having trouble with a specific pencil or pen, offer them something that you think might work better for them, just as you would offer them different sized lines or a different choice of paper.
Let us all know what happens.