不是A就是B：either A or B 不是A而是B：not A，but B

不是…就是…Either… or…，仅仅其中一个有可能，且说话者认为可能很大 不是……而是 not…but…

不是他去，就是我去————两个人都有可能去 不是他去，而是我去————他不去我去

The first one means that the two conditions can be one of reason. But the the second veto the former condition to confirm the later condition.

不是。。。就是If it's not one ,it's another one.There's a possibility.They all have a 50%(But there are exceptions in different contexts,exp:既然不是A，那一定就是B了。ji ran bu shi A na yi ding jiu shi B le。If it's not A,it must be B.The last possibility has been eliminated,so it must be another one.)不是。。而是Not this one ,but that one.Indicating that the former has been denied.The latter should be correct.There is no possibility.It means that the person who said it thinks the latter is right.exp：我要的不是沙拉酱，而是番茄酱。wo yao de bu shi sha la jiang er shi fan qei jiang。What I want is not salad dressing but ketchup. My explanation is not professional enough ,but I hope it can hope you.