Both the usbpre2 and the 7 series will operate in the field without a laptop, but the usbpre2 stands alone in offering you an interface if you wanted to record to your laptop.
I've used both setups (with a D50, but it's basically the same), and unless you have a reason (and there are some) for why you would need the extra robust nature of the 7 series setup, I'd pick the usbpre2>sony for two basic reasons. First, its cheaper, and second, you can use the usbpre2 as a DAC for your macbook to get better audio not only in recording, but also editing. Reasons to choose the 7 series are that you need a recorder that can survive in harsher elements, needs few connections/parts due to excess movement, or you need the security of redundant power options w/ fail over.