Different flavors of DAC implementation that have evolved as technology and the cost of components change.
- Yggy+ GS2 are the circa 2014 AD chips on the discrete Analog 1 board
- Yggy+ LIM/MIB are more current TI chips on an OpAmp based board
You wouldn't be "missing something" per se by going the cheaper route, but it would be helpful to know what your preferences lean towards sonically prior to jumping in.
And if you really wanted to go full bananas you could order the Yggy+ GS2 as well as the MIB boards separately for about the same price as the Yggy+ MIB by itself
Cover swapped my Yggys recently and the new chassis design is pretty straight forward, 4x screws and you're in!