There are far more knowledgeable people on this forum than me, but I'll give it a shot. The Spark was introduced in 2017 as DJI's entry level "Selfie" drone. It has four brushless motors (longer life than cheaper brushed motors), a two-axis gimbal mounted camera capable of 1080p high definition video and still photos. It has numerous programmed features that may be of interest for taking video while automatically maneuvering. It has an Active Track mode that allows the designation of a person/vehicle to be followed while shooting video automatically. The camera has auto and manual controls for settings such as ISO, shutter speed and exposure value. There are numerous 3rd party accessories available such as lens filters, gimbal guards, landing leg extensions, etc. It can be flown with a cell phone or even without one by using the Gesture Control feature. If used with the Spark remote control, it can be flown similarly to radio controlled airplanes while viewing what the camera sees on your attached cell phone. There are several 3rd party software apps that expand the capabilities of the Spark for things such as programmed, autonomous flight. Flight time is advertised as 15 minutes, but in reality is usually between 10 and 12 minutes. It weights right at 300 grams, so will require registration with many government agencies (i.e., the FAA in the U.S.). Spark also features a forward looking infrared obstacle avoidance system as well as a Vertical Position Sensor (VPS) for indoor flying and automated landing capability. It also has GPS.
The Mavic Mini (MM) also has four brushless motors, a 3-axis gimbal mounted camera capable of 2.7k high def video and stills. MM has fewer programmed features than Spark and no Active Track. The camera has automatic mode only. There is no Gesture Control and MM comes with a remote control that must be used to fly it (no phone control, but phone provides first person view of the drone as well as telemetry). Being new, there are currently no 3rd party software apps as yet. Advertised flight time is 30 minutes, but is more likely to be in the 20-25 minute range. MM weighs 249 grams which, being under the 250 gram threshold, does not require registration with many government agencies although the operator is required to adhere to the same regulations as drones weighing 250 grams or more. MM lacks obstacle avoidance, but has VPS and GPS.
Oh, I almost forgot, in the U.S. the drones can be flown per FCC regulations regarding transmitter power whereas most European countries require compliance with CE regulations. Net result is usually reduced range when using CE compared to FCC.
I hope this helps, but more official information and expert comparisons are available via Googling the net.