Have/want it both ways
Definition 1 of 2
Pro Tip 1/3Exclusivity
Emphasize that 'have it both ways' implies mutual exclusivity of choices or actions. 
If you want to be a vegetarian, you can't have it both ways by eating meat on weekends.
Pro Tip 2/3Informal Context
The phrase is casual and used in everyday conversation, not in formal writing. 
In discussions with friends, you might say, 'You can't spend all day playing and get good grades, you can't have it both ways.'
Pro Tip 3/3Decision Making
Use this phrase to highlight the need for making a decision when faced with two incompatible choices. 
Either go on vacation or save money, but you can't have it both ways.