In my opinion, both are important. Counting calories and ensuring you’re in a deficit is rule number 1 of fat loss. Meal prep is probably the best way for you to know exactly how many calories are going in.
In my current situation, I can’t meal prep, as I’m away from home for the majority of the week, but I’m still losing weight because I’m in a calorie deficit. Am I getting the right nutrients and macros on a daily basis? Probably not, and that annoys me, but I’m doing the best I can.
Given that you work somewhere that temptation is probably quite high, meal prep would be a great way for you to overcome that temptation.
I’d recommend you go over to tdeecalculator.net and input your information to find out your maintenance calories. From that, try and eat 3,500 - 7,000 fewer calories than that number a week to lose on average 1 - 2lbs of fat a week.
Hope this helps!