Fanfiction definitely has the most content and is where I do my posting - but I'm being gradually seduced away by AO3 - although most of my reading is done via mobile and this is actually quite poor on A03.

I've read several of your SPN/Buffy recs on tthfanfic (again via phone) and I was impressed with the layout.

My main reason for looking at Wattpad was to see if it'd be a good alternative to FictionPress which is definitely FanFiction's poorer sibling!