Clen will be more efficient, but cardarine cannot be undermined either.
with clen, you can only do short cycles, due to:
-fast receptor downregulation (have to titrate dosage frequently)
do clen for 2 weeks, don't go over 120mcg (start with 20mcg and work your way up depending on how your body responds)
2 weeks on / 2 weeks off works, but try to not exceed 8 weeks in total per year.
if you have any present heart problems (especially BPM problems like arryhtmia) forget it
if you are too paranoid, you can always constantly monitor your heart BPM + do EKG and just quit if it gets too overwhelming