Use water or milk in a tart crust?
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.