For me it depends on what effect you’re using or needing…
If it’s specific for only the one track then use it as an insert, and in this case you need to create a new stereo track and copy the mono track to the stereo track (I think that’s what you need because the track type e.g. stereo/mono/5.1 is fixed when you create the track…).
Or if you’re using an ambient reverb type then you’d be better off with a send, and then you may well use it for other instruments, which’ll help the track gel together too.
In terms of processing power, it makes no different using insert or send, but the point of sends is that you can route more than one track to the same effect, that can’t be done using inserts of course because you’ll use up your processing power by adding too many effects.
Mike.