Should I go to the gym 5 days a week to lift weights or to do cardio???

We have different energy systems: Aerobic and anaerobic. Running/cardio only works your aerobic system and your slow twitch muscle fiber. Whereas weight training takes you to Zone 3 (above 85% of your Maximum Heart Rate) and also works your fast twitch muscle fiber. But does not work your aerobic system too much. Therefore, a combination of cardio and weight training is the best routine. You can still go to the gym 5 times a week if you desire, but do weights 3 times a week and focus on HIIT/Aerobic the other two days.