Okay, just from the weather perspective: Average high temperatures for the day will be around 23 to 24 degrees Celsius on Mallorca or on Gran Canaria. But the chance of rain will be much higher at Mallorca with about 8 days of rain compared to one day of rain at Gran Canaria. Water temperatures will be about the same by that time of the year - 19 to 20 degrees Celsius for both locations.
Mallorca is more diving from the boat than diving from shores. On the Canaries it will be more shore diving. Of course you can dive from the boat as well. Personally, I find no join in diving in the Mediterranean. Almost no fish, no great vegetation, a bit boring from time to time. Chances to see something interesting might be better at the Canaries.
I like both islands. You can drive around on both islands (and on Tenerife and Lanzarote or Ibiza and Menorca) without any safety issues. Hotels, food and entertainment is great in all those places.
My advice: Canaries. And then the Balearics next time