I'm making a toffee apple tart that has a shortbread pastry as its crust, and the author of the recipe says you can use milk or water in the crust. Which would be better? Or do they each have their own strengths and weaknesses?
They are technically two different pastries. Shortcrust pastry made with water is called pâte a foncer, 'lining pastry'. Shortcrust pastry made with milk is called pâte brisée. Pâte brisée typically contains a bit more butter, and is generally lighter and flakier than pâte a foncer, which is conversely crisper and firmer.
So it's up to you really; both are very similar. The pâte brisée is more delicate, so if you have a 'heavy' filling, say, lots of fruits, it might be better to go with the foncer.