When Hans tells all his plans to Anna, she is on the verge of death. Only true love's kiss will save her. He leans in to kiss her then tells her the horrible truth - he is only interested in the throne.
Following this, he divulges his whole plan as she will be dead soon and it won't matter (and, of course, as it fills the audience in). He leaves her and likely assumes she will die as she is so weak.
When Hans meets Elsa out on the fjord, it has to be remembered that the ice that is engulfing the entire kingdom is her creation. She is unbelievably powerful. If he were to tell her his whole plan, she could easily overwhelm and defeat him. Instead, he tries to break her spirit by telling her that her actions effectively led to the death of her sister.
And it works. She falls to the ground, broken, and he has one glorious opportunity to kill her. Given he thinks Anna is dead and he was left in charge of the kingdom by her, this would eliminate both sisters and leave him in charge.
But the day is saved by Anna and the rest is history. Ultimately, it seems extremely likely that the reason he lied to Elsa was because it was his best way to defeat her. If he had told her the truth, like he did with Anna, he would have strengthened her resolve and risked defeat. Anna was so weak it didn't matter. Elsa was dangerous and wild - he couldn't overcome her physically, so targeted her emotions instead.