The one-word percent is standard in American English. Percent is not absent from other varieties of English, but most publications still prefer the two-word per cent. The older forms per-cent, per cent. (per cent followed by a period), and the original per centum have mostly disappeared from the language (although the latter sometimes appears in legal writing).
There is no difference between percent and per cent. Choosing between them is simply a matter of preference.
The city’s white population has a 6 percent poverty rate, lower than the white rate in every state. [New York Times]
There’s reason to believe that Gingrich’s support—pegged at about 15 percent in polling averages—is very soft. [Washington Post]
As an African-American member of the 1 percent, Tom Burrell sees himself as having a foot in two worlds. [Chicago Tribune]
Outside the U.S.
Treasury dealers were left holding 62.3 per cent of the $32bn three-year sale. [Financial Times]
It’s been a seller’s market in Greater Vancouver for 55 per cent of the last eleven years. [Vancouver Sun]
McDonald’s Corp gained three per cent after reporting January sales that were higher than analysts predicted. [Sydney Morning Herald]