Hi Ian, your options are:
trim that matches the thickest point.
or
mitre the edge of
tile (tricky)
if you go down the mitring route then ensuring walls are plumb is good practice but is a must IMO when doing an external corner, If you use trim then grouting will play a part in the finish, avoid being tempted in to thinking "that will be ok when grouted syndrome", it may looks acceptable at first if its hiding rough cuts but remember, grout depreciates in time so pay close attention to your external cuts both when cutting and when grouting.
Anyway, just realised you asked a specific question relating to trim, my preference is genesis square edge but for your
tile and situation I'd be tempted to source a corner
tile in keeping with the retro look of the metro
tile.
I'm currently having lunch and had a snickers.... Maybe that advert is true lol... Good luck