The 141s do not have cab lights. The 3 function decoder covers all the needs for these locos.
However, these chips do not support 'advanced consisting', so if you plan to double head your locos correctly then you would have to look at something like the TCS decoder. The 4 function decoder would be compatible with the loco (All Bachmann stuff complies with the NMRA standard) but I'm not sure if it does support 'advanced consisting'.
Of course if you do not plan to double head your Baby GMs then the 3 function decoder is fine. Or you could just give both locos the same number and run them in tandem.