Android is the leading mobile operating system among people who recently purchased a smartphone, but Apple's iOS still holds a slight lead over the Google-developed OS.

About 40 percent of those who purchased a smartphone in the last six months opted for an Android-based device, according to Monday data from Nielsen. But iOS still holds the mobile OS lead with 28.6 percent of the market, while Android has 25.8 percent. BlackBerry has about 26.1 percent.

Given such close numbers, it can be difficult to establish a clear winner, Nielsen said.

"In other words, RIM remains statistically tied with both Apple for first and Android for third," the company said in a blog post. "Apple's clear lead over Android notwithstanding, this race might still be too close to call."

One thing is for sure, Nielsen said; smartphones are growing in popularity. In November, 45 percent of buyers chose smartphones over feature phones.

Earlier today, NetMarketShare reported that Apple's iOS saw the greatest overall usage share gain last month in terms of those accessing the Web on its platform. Web use on the Android platform, however, grew at the fastest pace.