in fact it's very simple.
you must know what "subjunctive" is, right?
When we want to express some emotion, we could use the subjunctive.
For ur question, if it's a subjunctive, plz use 还是. Otherwise, plz use 或者.
In chinese, we say: 当说话者表示疑问、假设、主观愿望、怀疑或推测时，可以用虚拟语气（subjunctive）
for example, 你要苹果 还是 香蕉？（疑问）
for the normal case, we use 或者.