There are resource issues either way, in both cases you have to store state. With threads this can quickly become expensive if you keep the stack size for all t

There are resource issues either way, in both cases you have to store state. With threads this can quickly become expensive if you keep the stack size for all those threads at the default amount (a whopping 48K if memory serves). I would say it's easier to use smaller amounts of memory with event-driven programming, but you still have to keep some data around per-client. Of course, if the situation is very asynchronous (seconds instead of milliseconds) then you don't necessarily need all of this hanging around in RAM.