Despite VIA's insistence that Nano really is faster than Atom, the results of our real world tests were still surprising. The sheer extent to which Nano beats Atom in nearly all our tests is quite compelling. Indeed, on this basis alone, there's no-contest.
Subjectively, in general use, it's the same story as well. While general desktop work on Nano feels responsive and snappy, there's a constant sluggishness when using Atom - even just moving a window can result in staccato motion.
Now, you could argue that Vista isn't a fair operating system to use for testing as it's not what Atom or Nano is designed for. Nonetheless, the fact of the matter is that using Vista on a Nano is possible whereas on Atom the experience is less satisfactory.
The one big caveat here is the thermal aspect of the two processors. While Nano's performance is higher, the simple fact of the matter is the processor potentially kicks out more heat than a really small device like a UMPC would be able to cope with. Indeed, VIA suggests the 1.8GHz L2100, with its TDP of 25W, won't be finding its way into portable devices at all but rather the 1.6GHz L2200, or slower, chips will be used in these devices.
Essentially, what would seem to differentiate Nano and Atom is that they are aimed at and perform in accordance with different markets. Nano has the performance, at peak, to power through the sort of tasks a notebook is expected to perform yet also has a very low idle power usage, making it perfect for this market.
However, Atom was designed for the UMPC/MID market and it's here, rather than in notebooks, that it should more often find itself. It's very low TDP means it can fit in very small devices, which the likes of a Nano would overheat in, and the simpler single tasks that a UMPC type device would perform are also more suited to its design.
Ultimately, though, it's down to OEMs to use these platforms to their best effect so we could still be seeing great devices that use either, it just depends what you want from said device. Now, hurry up VIA/OEMs and get us a Nano powered Mini-Note to look at so we can do some like for like comparisons.