What factors influence how people choose a channel for international money transfers?

Very open-ended question, as the variables differ from territory to territory. In general, some of the common factors are:

• Strong endorsement (recommendation) from someone they know they can trust or have a rapport with.
• Receive side recommendation
• Cost, speed and ease
• Familiarity

Other factors do come in play, but these (IMHO) are the most important ones.