The USB Pre has very similar preamps to the 7-series, but adapted for 5v power supply. It has about 2dB lower EIN than the mix pre, and about 10dB lower noise ADC than the mix pre. Depending on the sensitivity of your mics, these figures may not make any difference to your recordings.
Re AES or optical, AES is the studio standard but it's essentially the same data being transmitted so there's no sonic difference between the two. AES via XLR is a much more robust physical connection though.
I'd go with the mix pre. I found the USB Pre to be a unuseable in the field due to its rear panel dip switch settings and lack of stereo link options. It was a champ in the studio, but not built for anything else. The mix pre preamps are the classic sound devices lundahl transformer pres, and if you can live with the extra 2dB noise over the USB pre, they sound exactly the freaking same as any other half decent preamp out there.