I would bet pretty solidly that Diomande will be the starting striker (albeit maybe not in friendlies if he doesn't get his work visa soon), with JJ first off the bench. For one, it's tactically sound to keep the guy who can play all three front positions in reserve. For another, as a pure goal scorer, I think Diomande is valued more (probably correctly) by Bob than JJ.
JJ has huge skills but clearly not alot of confidence as a fox-in-the-box, because he's never done that before. He's always been coming in from wide, cutting in on his strong foot or beating the backline. He's used to hanging off a single defender's shoulder and meeting a through ball first-time with great accuracy to the lower corners.
Diomande is a traditional forward. He's strong, he can finish with both feet, he was very fast although I expect that's tempered somewhat. He can finish with his head and he knows how to create space in the box when it's crowded, as well as where he should be at the right moment. The service doesn't have to be tailored.
I actually expect he'll protect Insigne a little, particularly if we're doing well, and JJ plays that left wing forward position naturally. It's what he played most of the time in Spain and Poland. So he'll get time there, too. And if Diomande craps out or is looking poor, he has a different look he can immediately go to.