I don’t know.
On one hand, Boukan no Rebellion really feels like it should be read after watching Steins;Gate. Otherwise, you wouldn’t understand the whole drama around Suzuha, you wouldn’t understand the bond between Rintarou and Mayuri, you wouldn’t understand the pain and torture Rintarou went through after Mayuri died fifty thousand million times.
On the other hand, Boukan no Rebellion really feels like it should be read before watching Steins;Gate. Otherwise, all the mystery around Suzuha is already spoiled, all the suspense around Mayuri’s death is unsuspenseful, and all the drama around her dad is wasted.
Read and watch them simultaneously. Send a D-Mail to your past self to tell them to do that. Oh, and tell your past self to tell me to do that too, will ya (although then, the me who’s writing this post presumably dies. forever. not even in a well off the coast of finland…)?
Two anime posts in a row because Steins;Sub just released the episode and the next episode’s going to be released tomorrow… and of course, some people probably got posts about the next episode out /already/, before it even aired…