Most definitely Instant Book. Instant book allows qualified applicants to book your home at any time without going through the back and forth of initial contact. Taking same day bookings allows last-minute travelers the option to book - whether they are Instant Book qualified or not. Since we take same day bookings and have Instant Book turned on, we've had only 3 same day bookings, but more than 50 Instant Books.
To avoid the situtation you found yourself on your husband's birthday, you can set same-day reservations to a "cut-off time". Ours is 12pm. But frankly, I don't think you'll miss out on a lot of bookings if you don't take same-day. In my experience, people who book last minute are typically more work because they are poorly organized (thus the last minute booking). But that may not be your experience.
Good luck.