I'm using the Sound Devices USBPre 2 and I'm familiar with the MixPre-D. Either can be used for ENG though powering the USBPre 2 in stand-alone requires a USB power source. The USBPre 2 used to be SUBSTANTIALLY cheaper but I guess Sound Devices clued into the fact that it was eating into their ENG sales and kicked the price up.
The MixPre-D has a dandy bracket available for mounting to a rig or camera, tape return, and internal batteries. The USBPre 2 is richer in I/O. Both are extremely robust, and very high quality as would be expected of Sound Devices. $$$.