double or single stroller? HELP!

kirsten • Married for 4 years. Currently have a one year old princess💜 and a 1 week old prince💙
When my son arrives my daughter will be 16 months. I am debating getting the Graco Ready2grow double stroller for both of them, but at the same time I want something just for him in case I dont have both kids with me. What are your thoughts? double or single?