They don't provide the same info (though serebii does what bulbapedia does). Bulbapedia does what it does billions better than serebii does it so people only use serebii for news about pokemon. I don't like to spoil myself too much about upcoming pokemon stuff so i only use bulbapedia.
Turtles are amazing. No lie. Just like how Golbez is the best thing to happen to gaming. The Official Theodore Harvey of the Dissidia 012 board, i guess.
I use them about equally, maybe with Bulbapedia seeing a bit more usage due to the site looking better than Serebii's. Like Mugiloko said, one site usually does something better than the other, but they generally have about the same information, more or less. (Mostly because Bulbapedia steals stuff from Serebii. >_>)
TOTALLY NOT HOME ROWED want some funeral to go with that cake?