While these two lights are in the same high class in terms of quality in general (both could be considered close to "best of class" lights), you are correct in assuming that they are not in the same class. The Arc isn't "just a little key chail light"- it could compete with a minimag several times its size- but it does use only a single 5mm LED, and even though that LED is the lastest, brightest version, which in turn is overdriven, it could never compete with a fully driven Luxeon LED (over 10 times brighter). Really, it depends on your needs- the Peak is very compact for a light that will outshine a 4D Mag, but it still would be too big for your keychain- the Arc you would always have with you. Of course, the best light is the one you have when you need it.
If you are comfortable with a slightly larger light, and would be willing to up your funds by $20, I'd also encourage you to look at the HDS EDC Basic 42. This light is more of a "jack of all trades" since it has a nicely balanced primary brightness level, but can dim to two lower levels or one much higher level as needed.
Also, for about the same price as the Arc (around $45 shipped) you could get the Fenix L1P which offers excellent quality for the price (probably a better value than any of the lights mentioned thus far). It won't be as bright as the CR123 lights, but it will still blow away the Arc and has the bonus of using almost any kind of AA battery.
Welcome to CPF! The motto here is "buy them both" so hold on to your wallet!
