Well, first off, "UI" stands for "user interface". That's the way the software is programmed for the user to control things--the combination of on-screen things clickable with a mouse, the displays that give you information, keyboard shortcuts, that sort of thing. The UI makes a big difference to the speed and experience of using the software but has nothing to do with audio quality.
The MBox and Digirack are different forms of audio interface: devices which convert from the analogue world of mics and such into digital signals that a computer can use. Since they contain both microphone pre amps and analogue to digital converters, such things can indeed make a difference to the sound, especially the pre amp which is second only (but a distant second) to the microphone itself in determining how the recording sounds. However, as you've discovered, just because they CAN make a difference, doesn't mean they always do, especially when they come from the same "family" of products like the MBox and Digirack.
I guess the other thing to say is that, to hear subtle differences between pre amps and interfaces, you also need monitoring good enough to reproduce these differences.
