How big of a booth are you thinking of making? For miniature airbrushing I don't think it makes a huge amount of difference, if you just have a decent exhaust fan to pull away the air immediately in front of you, then fresh air pulls in from where ever. That's what most hobby booths are, just a small semi-enclosed space a few square feet large with an exhaust fan, and fresh air just comes from the rest of your room.
Cross draft vs down draft is more of a consideration when building a very large booth, like, the size of an entire room, with the intention of spraying things like furniture or cars.
For a hobby sized spray booth, I think the main benefit is having your exhaust vent so it's pointing directly upwards as it leaves your house (assuming you're exhausting instead of just filtering and recycling the air). Reason being that when wind blows it could blow into your exhaust vent, overpowered your fan, but if it's pointed upward it'll tend to assist the fan even on a windy day.
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