Both maybe? Choux formulas are pretty standard in the proportion of flour, eggs, liquid and fat. Where they sometimes differ is in the type of liquid they call for: some say milk, others say water, some say a combination. Does it make a difference?
It does. As in most baked goods, milk acts as a tenderizer. The milk solids and fat disrupt gluten networks and create a softer product. That said, there’s plenty of fat in choux batter to begin with, so a little more milk fat won’t make that much difference. However milk also contains lactose and proteins, both of which will help the choux puffs brown in the oven.
So milk will have three effects: a (slightly) lower rise, a (slightly) more tender texture and a (noticeably) darker crust. If you want to mitigate those first two effects, you can take away a tablespoon or so of the butter, which will bring you more or less back to where you started in terms of fat and milk solids content.