Do you think Clenbuterol is more effective or has fewer side-effects than Ephedrine or Amphetamine?

More effective (for weight loss) and less side effects than amphetamine (at weight loss dosages of the latter).

More effective but with more sides than ephedrine (Clen isn't a toy!). But at doses needed for ephedrine to compete with Clen in desired effects ephedrine has more sides. Essentially both substances work via the same mechanism (beta adrenergic receptor agonists) but Clen is much more specific to beta-2.

Clenbuterol really comes into its own when used for cutting (losing weight after already gaining a lot of muscle). In this situation losing weight is already easy enough - aside from the energy use by the muscle tissue the bulking cycle has probably already involved force feeding - eating less is the default. The trick is to lose the fat quickly while minimising the loss of muscle. The mild anabolic effects of clenbuterol (through mechanisms completely unrelated to that of androgenic substances) partially offset the overall muscle catabolism.

Clenbuterol is also rather handy for cardiovascular and endurance performance - it's an asthma drug after all. If a cyclist is banned for doping and it isn't for EPO it is probably for clenbuterol.