You can't eat your cake and have it 两者不可兼得；鱼与熊掌不可兼得

【例句】
Roger can't make up his mind whether to go to college or get a job. You can't eat your cake and have it, too.
罗杰下不了决心是上大学还是找工作，两者不可兼得。

You can't eat your cake and have it, you must make a decision.
鱼与熊掌不可兼得，你必须做出决定。