Likely Proof of Work, but using something that is more ASIC-resistant (look up Cuckoo Cycle) and makes mining a more even playing field. Proof of Stake is still very cutting edge and once you go down that road, you can't really go back. So you have to make sure you have distribution of tokens etc. really locked down sensibly. That's what worries me about Ethereum switching to Proof of Stake with such centralized distribution (made even worse with ICOs pushing thousands of ETH into single addresses).