In my personal opinion, it's a no brainer. Cardarine is the clear winner. It is way more effective and causes way less side effects then clenbuterol.
Clen simply raises metabolism. It does not differentiate between muscle or fat. Clen might raise your metabolism an additional 100-200 calories per day. You could just easily do an extra 15 minutes of cardio. I have used clen in the past and I always felt it did more harm then good. It caused muscle cramps when I was training and it really negatively impacted my sleep.
There are better alternatives that directly increase fat loss, with no risk of loosing muscle mass and no side effects like insomnia or muscle cramps. Cardarine gw is going to be your best option. It directly increases fat loss, endurance and recovery. It is not a stimulant or a hormone and is pretty much void of side effects.
I haven't used clen myself in years and don't recommend it to my clients at all anymore. If they are trying to burn fat, I recommend cardarine and maybe a good nutrient partitioning supplement like n2slin from n2bm