Of the world's two largest international financial centers, London is warming up to the renminbi, but Wall Street is running behind.

Recently, the former mayor of New York City Mike Bloomberg congratulated President Barack Obama and President Xi Jinping for a number of economic steps, but in particular "advancing a mechanism to trade the Chinese currency in the United States."

Bloomberg responded to the conclusion of the recent US-China Summit in Washington. According to the White House release "the United States and China look forward to continuing to discuss mechanisms to facilitate renminbi trading and clearing in the United States."

It was the sole and short mention but quickly noticed. As former mayor of New York City and its legendary financial district, Bloomberg has reason to take notice of such statements.

For years, New York City's Wall Street and London's City have competed for the role of the financial capital of the world. Seen purely in terms of size, the New York Stock Exchange has a market capitalization of close to US$18 trillion; and NASDAQ US$7 trillion. The market cap of the London Stock Exchange is over US$6 trillion.

Size matters but it's not everything. Both Wall Street and City own large international full-service centers with advanced settlement and payments systems. The two represent large economies, with deep and liquid markets. Both are also seen to have relatively high legal and regulatory standards.

According to the annual Xinhua-Dow Jones index, New York has been the top-ranked center since the 2010 launch of the index.

The two are followed by Singapore, Hong Kong and Shanghai. According to the semiannual London-based and Qatar-sponsored index, New York has just lost its leadership to London and both are followed by Hong Kong, Singapore and Tokyo.

While New York City remains the largest center for trading in public equity and debt capital markets, London maintains the largest trade surplus in financial services around the world.

Historical experience suggests that leadership in international financial services requires these hubs to remain close to global markets and the innovation frontier. Yet, unlike Wall Street, London has embraced the renminbi.

Last spring, many countries in Asia and elsewhere joined the China-proposed Asian Infrastructure and Investment Bank, while Europeans stood aside.