Clenbuterol and Ephedrine are both beta-receptor stimulators, however Ephedrine can stimulate a range of beta receptors (the beta-2 receptor is the one that causes CNS stimulation IIRC) which is why it causes more CNS stimulation. This makes it raise the heart rate more then clenbuterol. Clen is much stronger at its binding of the beta-3 receptor, however, which is why the effective dosage is extremely lower then the effective dosage of Ephedrine.
Which one is a better fat burner? You will hear things in defense of both substances, however its more widely accepted that clenbuterol is "better".
I do not mean to make assumptions, but I think all you really need to do is follow a solid diet and cardio plan to lose fat. IMO from personal experince with clenbuterol, it is highly overrated. It is only useful when you are trying to burn off the last 1% or 2% of fat that just doesn't want to come off.
Otherwise, just stick to an ECA stack, diet, and cardio.