What I look for in my factions tend to be multi layer tricks, the ability to take out key enemy models at my whim, preferably speed and if possible the ability to deny damage either because of defense or armor. (cryx, skorne current warmachine factions)
When I have demoed in the past for games I try to get armies that both show the basics of the game and have models that will stop a passer by to make them ask what the game is.
I did this with a game called spinespur by getting Hack(Jason like character) and having him fight Sex star six(kiss looking model), then supporting each with a group of the basic thugs. The game also had a pope like character and a bondage penguin that would sometimes make their way out into the demos too.
So the choice of arcanist seems like a no brainier for demo reasons since I would either be picking russian hottie and ice beasts, bad ass looking guy with awesome beasts of destruction or steam spiders. Any of those would cause people passing to at least take a second glance.
On the other side should be something else equally as kool and evocative. Either of the two resurrectionist would be very eye catching but so would the gunslinger esc feel of the death marshel( that just scream at the king seris hero to me). Maybe that is just me