These days the Music of the Baroque's "sidebar series" seems to be livelier and more adventurous than its main series. A case in point: next week's revivals of Vivaldi's Credo and Pergolesi's Stabat Mater. Both works are rarely performed, even though the Pergolesi qualifies as a crowning achievement of the Baroque era. Credo is noteworthy as Vivaldi's only known setting of the Nicene Creed; scored for chorus only, it conveys fervent devotion through a series of contrapuntal utterances. The Stabat Mater, on the other hand, turns a liturgical subject--the mournful prayer of Mary at the foot of the cross--into the stuff of theater. Pergolesi (1710-1736) was the Baroque's Mozart: he wrote much--especially comic operas--and died young. Largely ignored even today, he's been admired by connoisseurs such as Rossini (who wrote the other great Stabat Mater) and Stravinsky. This presentation features two of the city's finest women singers in parts originally intended for male soprano and alto (the church forbade women from performing in public): Patrice Michaels Bedi and Karen Brunssen, who have both been coached by MOB leader Thomas Wikman for a long time, and who are both entering their prime as soloists. Monday, 8 PM, St. Nicholas Church, 806 Ridge, Evanston. Wednesday, 8 PM, Notre Dame Church of Chicago, 1336 W. Flournoy; 986-3236.