As far as Gran Canaria is concerned, it is generally busier in the winter months than the summer months. In the summer it is too hot (average 25-30c) for many Europeans but in the winter it is warm (18-22c) without being too hot and this appeals to many Europeans who would otherwise have to go south of the equator to get warm weather. Rain in rare but happens and you could get overcast days in the north of the island, particular around Las Palmas. The south is better if you are after sun, sea and an active nightlife.
Whilst many people sunbathe, and it is far more difficult to find a sun lounger for hire on the popular beaches during December/January than July/August, the Atlantic is perhaps a wee bit too chilly between November/April but swimming pools (even unheated ones) are inviting.
I have spent several Christmas/New Years in Playa de Ingles/Maspalomas and the place is buzzing, night life is in full swing and most bars are open (some close during the quieter summer months). New Year's Eve in the Yumbo Centre is something that you should experience. Compared with the pace in December/January, July/August is almost like a funeral.
On the downside, hotel prices creep up in the winter due to increased demand and it gets dark by 19:00. They start to stack all sun loungers/sunshades on the beach by 16:00, which seems early when you are on holiday. The Canaries used to be a cheap and a duty-free zone (bottle of vodka GBP 4, 20 Marlboros GBP 1.50, meal with wine about GBP 8) but all that changed about 10 years ago. Prices are now the same as mainland Spain.
MOL on SRB's latest attack at BA: "It's like a little Chihuahua barking at a dying Labrador. Nobody cares."