There are several big threads about this over at CPF. Basically the Arc has somewhat more efficient electronics and will get you more total lumen-hours from a battery. The Peak "snow" leds have much more neutral tint. Also, you can buy Peaks online and they've always been able to ship promptly, while Arc is usually backordered (like now). Arc has always made excellent lights but has had trouble shipping product out the door, leading to its old incarnation's collapse. Hopefully the current incarnation will get the bugs out of its process soon and be able to fill orders promptly.
Either way, by most people's standards (i.e. people who are used to minimags and whatnot), the performance of both these lights is amazing. Whether they're worth the expenditure is subjective. I'd say if you already have one, it's not worth "upgrading" to the other, unless you're a flashaholic and are willing to buy lights just because you like them.
Note also that you can buy Photon-style lights for well under a buck apiece. Ten of them in your BOB may in some circumstances be more useful than one AAA-powered light.