ARM chips have actually been around for longer than the atom chips, they are mainly used in mobile devices (phones etc) and their core architecture is completely different (or revolutionary, depends how you look at it) to a conventional cpu like an atom.
What is better? well the ARM is better for mobile devices, though ive not seen any ARM chips working in netbooks etc so cannot comment, but its worth looking at the technology behind ARM chips as it could be common place very soon.
It's always good to see technology developing new ways to save energy and still produce results. The ARM/TEGRA2 stuff seems good, although many users of this technology say Atom is more powerful and will outperform the ARM technology based purely on performance alone.
With ARM, what I have noticed is, with the android platforms, less resources are used so, of course, one can achieve a little more with the headroom compared to windows/linux/osx based x86 chips. As far as a revolutionary development, I have to admit I'm a bit cynical. Maybe in some time newer versions of this ARM architecture it will become advanced enough to compete with x86 architecture, but as far as I can see, it is merely a tactic to get us to buy more mobile computing devices using a slightly different setup.
As for atom, the performance is comparable to pentuim III - so we are still with no mobile devices that can deal with high end graphics and gaming.
There are examples of ARM/TEGRA chips in netbooks (like this comparison test: http://www.youtube.com/watch?v=Tk8DTx0Xnfw). Like I said, I am looking at how this 'advancement' actually benefits the consumer (aside from the noticeable energy saving aspect)
Benchmarks would still be a good thing. I wonder why, with all the CPU enthusiasts out there, there are still no hard statistics comparing the two platforms. It would allow the consumers and insight into the actual progress technology is making. Until that happens, I'll remain as cynical as before.