They work differently for different people. Oh how cliche is that.
Some guys on here have claimed clen made them drop 20 pounds in 3 weeks without changing their diet...others have said they were up to 150mcg/day and shaking like crazy with no results at all.
Ephedrine is more reliable in that everyone on ephedrine loses weight if they're being responsible with their diet.
I've done both lots...and I'll say clen is more powerful, but dont take it if you have to write much, your pencil wont sit still.