The biggest differences: BackTrack + Mic features a microphone and a 1/8-inch headphone output, BackTrack does not. Both feature similar features, though, including variable sample rates and bit depth, which are shown here:

BackTrack + Mic BackTrack Sample Rate Bit Depth
24 hours 12 hours 11 kHz 16 bit *
12 hours 6 hours 22 kHz 16 bit *
8 hours 4 hours 32 kHz 16 bit
6 hours 3 hours 44.1 kHz 16 bit
5 hours 2.5 hours 48 kHz 16 bit
4 hours 2 hours 44.1 kHz 24 bit
4 hours 2 hours 48 kHz 24 bit

*These settings are not recommended for mic’ed full-range audio but may be sufficient for direct guitar.