Speed and cost?
Not even sure they are gains over 6mm TBH. Cost for an army at these sizes are often a one off purchase with the advantage being you often buy two or three ‘sides’ over time as the cost makes it viable long term.
3mm painting time is similar to 6mm for me and again it’s normally a one off compared to the game time.
For me, it comes down to range choice and if they have to options I want – more important in fantasy than ‘traditional’ human armies.