The most intriguing feature this year at Ravinia's annual A.A.C.M. offering may be the contrast between two remarkable drummers. Reggie Nicholson, with Mwata Bowden's quartet, is a master of free motions, of subtle interplay using a wide dynamic range, of infectious rhythms and organically growing swoops of sound; Avreeayl Ra, with New Horizons, is all excitement, mixing African, Latin, and jazz rhythms with power and never failing to bring out a band's best. These two remarkable drummers play with two remarkable bands: Mwata Bowden himself is a treasure, playing rough-hewn baritone sax with straight ahead force, veering naturally to expressive extremes; on clarinet, his rich sound suggests a 20s New Orleans player doing free jazz, and in recent years he's added a new dimension by playing dramatic, lyrical tenor sax. His horn partner is Ari Brown, whose own tenor is most fluent with ideas. The Bowden group does nothing but outside jazz, while New Horizons is a free-bop quintet. Its trumpeter, Ameen Muhammad, and its trombonist, Steve Berry, are two of the several fine brass players to emerge in Chicago in the 80s. Ernest Dawkins leads the band, plays a tough, angular alto sax, and composes attractive songs, half hard bop and half outside. Interestingly, the forceful, big-toned Yosef ben Israel plays bass with both groups. Tuesday, 8 PM, Bennett Hall, Ravinia Festival, Green Bay and Lake Cook Rds., Highland Park; 7284642. Bowden and Sound Spectrum will also play Monday, 8:30 PM, Hot House, 1569 N. Milwaukee; 235-2334.