Without having used it:
Red-Purple is still better. Mainly due to the fact that if you as shotgun user really try applying expose you can use bombardment. Which is far more consistent.
That said if you specifically aim for maximum uptime on blue you should be able to reach I believe around 93% uptime on expose. It does however probably produce a little indirect damage bonus when being solo (like in da) since you will have expose pop up decent lengths from time to time.
I wish I could test it but getting the howling dog shotgun to comparable values as my spes will take ages.