I believe Sofia Vergara will attract more attendees than Reese Witherspoon. In my opinion, when females are trying to get their significant male partners to accompany them, the males will see Reese as "cute" but Sofia as "hot". Sofia was also just granted a star on the Hollywood Walk of Fame.
Both Sofia Vergara and Reese Witherspoon have a strong fanbase. They are also both very talented women. From what I have seen in previews they also seem to work well off each other and have a good chemistry on screen. I think this movie ended up with a winning combination in putting these two actresses together.
I think people will attend mostly to see Sofia Vergara, because she has become increasingly popular recently. Reese Witherspoon has been popular for awhile, but doesn't seem to moving up a lot in popularity like Sofia has been. It seems to me that Sofia Vergara has a fresher, more current persona than Reese WItherspoon, which will also lend to people going to the movie primarily to see her.
I believe people will be compelled to see "Hot Pursuit" because of Sofia Vergara. Reese Witherspoon has received lots of bad press because of entitled, unappealing behavior. I think people generally have a negative opinion of her right now. Sofia Vergara has a very likable personality, and her bombshell appearance certainly works in her favor to win her more fans.