SHF is better in terms of visibility for loco pilot for most alcos and EMD's ( except WDP3A, WDP4D and WDG4D which have dual cabs). But LHF or SHF allotment is usually based on loco availability. Earlier all steam loco sheds used to have a turn-table to turn the loco around. Some diesel sheds also used to have this to ensure that the locos are always kept in SHF mode. But these turn-tables are not present every-where ( only in major diesel loco sheds). So, if a train arrives at a station with diesel loco in SHF mode, if the same loco is given to a train in the opposite direction, it will be LHF mode.