Cosplay both. They're both very fun characters to pose as, and I would know, as I have cosplayed both of them before. Tohsaka is more fun if you have an Archer to pose with, though. Rikka is newer, so isn't as recognizable (but people will appreciate her more if they know the series, since she's not cosplayed as often.)
On another note, your title is deceiving as I was hoping there would be a chuunibyou photoshoot being planned...but then I was confused when I read the fate stay night part. lol what