How effective is the fistula plug? My surgeon said that according to his experience, it's about 70%. Is it worth trying? Or go with anal fissure?

Different. Anal fissures and fistulas are different problems. Fissures are tears, fistulas are tunnels or tracts. A fissure is not treated with a plug. Anal fistulas can somtimes be treated with a plug. It is worth trying, If it doesn't work, other surgical options are available.
Reviews have been... ...Mixed, but more positive than negative. The decision as to what treatment approach is right for you should be jointly made by you and your physician (who knows you).
Less than 50 percent. Even in the best of circumstances, long tract, prior seton to lessen granulation, i think that the best one can expect is 50%. If the fistula is complex, it can be part of a more complex repair or if this is transphincteric with a high risk of incontinence, it may be worth the effort and cost(many insurances will not cover the plug). I am not sure what you mean by "go with anal fissure".