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Whichever one they want to paint and play with.
That's the best answer really. What you might try is heading over to the
GW store and opening a new tab for each model in each faction and then go through and close any tabs on models you don't like. See how many are left and what's left after you do that.
You might find that you like the idea of the army, but on a model front you really only want to own/build/paint one or two etc...
Both are solid choices. Custodes have the advantage of being a more elite based army in general. So you'll have fewer models on the table
That said Tyranids can go many ways and are very diverse. You can go with very few big models or you can go with a middlerange or a mix or a swarm of smaller ones.