If I had to choose between those two I'd have to say Nikki. But I don't think either of them is a bully.
The real bully in that house is Jayne. She keeps saying she wants it to kick off with Ash and if Nikki goes this week then it will, for certain. I can also see her gloating straight away, in front of Ash, if Ash goes this week. At least the others will then see her for the vile odious creature that she is. Both her and Nikki have repeatedly tried to turn other people against Ash, which thank goodness hasn't worked. Jayne also keeps having little digs at Ash but so far Ash hasn't risen to it.
I don't think her fueling Nikki's hatred of Ash is doing Nikki any favours either.
I honestly don't know what BB are thinking this week