Very interesting pairing for sure!
Their positions are similar because they are both considered 'aces' of their groups and with Acchan leaving, Mayu's getting a bigger chunk of the 'face' role as well. Jurina has the 'face' role with SKE48 since the start. Both characters are highly similar, they're aiming for the top and they are very competitive.
They're both big parts in the 'next gen' AKB48, but I'm really curious, and maybe a bit worried, how this will play out.
Either they'll become rivals. Both fighting for the top. This could get out of hand meaning one of them could drop out.
Another option is that Jurina accepts Mayu's her senpai and follows her lead. Then eventually, when Mayu graduates, Jurina can take over. This is possible, since J's that much younger than Mayu.
I'm not sure that there's a option between best friends forever and ultra rivals
The best thing about them might be that their images would appeal to different fans.
Mayu is more like Acchan: girly, cutesy, etc.
Jurina is more like Takamina: bit more outspoken, boyish, cool
Because of that they could really work well together as the new 'two top' of future AKB48.