“I’d rather short ether.”

Andrew Left, the activist short-seller behind Citron Research, told CoinDesk on Tuesday that he thinks both bitcoin and ether, the cryptocurrency of the ethereum network, are “bubbles.” If he had to choose one, though, he’d bet on ether’s price to fall.

Left is known for writing explosive reports into companies like the Canadian pharmaceutical firm Valeant, arguing that investors should short them.

His comments came a day after he fired a Twitter broadside at Square, a payments company that has been adding support for bitcoin buying and selling. He declared that the company’s share price is likely to drop to $30, from just under $49 prior to the tweet.

“WallSt drunk on Bitcoin nonsense, SQ-Cash to BTC trading has been insignificant,” Left wrote.

Square is scheduled to report its latest earnings data on Wednesday, and if “they’ve earned nothing from crypto, don’t be surprised,” Left told CoinDesk.

Left has taken aim at other investments related to cryptocurrency in the past, but he told CoinDesk that each had an “individual reason.”

Addressing bitcoin’s merits in and of itself, he acknowledged the appeal of a decentralized asset or a kind of digital gold, commenting: “I dig it.”

He’s not so sure that bitcoin will win out, he said, but as mentioned before, he would gladly short the second-largest cryptocurrency by value. Left dismissed a bullish argument for ether – that its price will grow because entrepreneurs can issue tokens and build decentralized applications on top of the ethereum blockchain – using the analogy that Oracle doesn’t own part of the businesses that run on its own servers.

In his view, ether bulls use a “circular argument” – that is, “it’ll go up because it’ll go up.”

“I think both are bubbles,” he said of bitcoin and ether, but “if I were going to be wrong about one of these, it would be bitcoin.”

Left has been shorting stocks full-time since 2001, and has said he’s made money every year. Short sellers profit when the price of a stock (or another asset) falls; they accomplish this by borrowing shares, selling them, and buying them back later to return to the original owner.

The Citron founder is most famous for correctly predicting the fall in shares of Valeant, a Canadian pharmaceutical company that has been at the center of an accounting scandal.

Andrew Left/Citron Research image via YouTube