Most spreads take time for theta to decay 20-35 days. The risk of legging in is that you could make 2 bad trades in a row if you are wrong twice, or that you may fill on the sell side for lower than you would have sold in a single transaction. When you have a stock that moves as quickly with substantial moves, leggin in is my preferred way.
Why, because if I buy the long side and I am right, then there is never a need to short against it and pay comms for that leg. I just let the longs ride.
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