A word about alternate selections: During Ordinary Time (the numbered Sundays after Epiphany and after Pentecost), the Revised Common Lectionary provides for two streams of readings. In the semi-continuous stream, the Old Testament readings flow from week to week, telling the OT stories. In the typological stream, the Old Testatment readings are chosen to relate thematically to the Gospel reading. A worshipping community should select either the semi-continuous stream or the typological stream, and stick with it throughout the entire season. The psalms are not meant as alternative selections, they go with the stream of readings. So you use either the first psalm or the second one, but don't flip-flop around from one week to the next or you miss the entire point of having a lectionary.