It should come as no surprise that Apple and Android are the two "most desired" smartphone systems, according to a new Nielsen survey of mobile phone users. But the results vary a bit based on the people who were surveyed.

Among current smartphone owners questioned by Nielsen, the survey results out today show that 35 percent are looking to buy an iPhone as their next device, while 28 percent plan to go for an Android phone. But among those who currently own a feature phone and want to upgrade to a smartphone, 25 percent are eyeing an iPhone and 28 percent will likely opt for an Android device.

To define the difference: smartphones run a third-party operating system, such as Apple's iOS, Android, BlackBerry, Windows Mobile, and Nokia's Symbian, and they support third-party mobile apps. Feature phones use a proprietary OS, are typically unable to run third-party apps, and usually have other limitations.

Among age groups, the iPhone proved slightly more desirable than an Android phone to people 18-24, 25-34, and especially those 55 and older, according to Nielsen. Those 35-54 preferred an Android device as their next phone by a slight margin. And by gender, women favor the iPhone as their next purchase, while men are more interested in Android.

Smartphone owners were more decisive than feature phone users about their next device in terms of operating system. Among those who presently use a feature phone, 25 percent said they were "not sure" what their next phone would be, compared with only 13 percent of smartphone users who were equally uncertain. People 55 and older were also less certain than younger users over which mobile OS they'd want for their next phone.

Overall, 29.7 percent of all U.S. mobile subscribers use smartphones, compared with 70.3 percent who still have feature phones. The most common smartphones platforms in the U.S. are the iPhone's iOS and the BlackBerry OS, each with 27 percent of the market, while Android now holds a 22 percent share, according to Nielsen's data.