I'm new to doing my own quilting on my sewing machine-- so I've been doing image searches to find ideas for the FMQ or straight stitch designs. I think you can go one of two ways with split rail quilts; either a curvy overall pattern which is a complete contrast to the simplicity of the rectangular pattern of the quilt top OR emphasize the geometric patterns with stitch in the ditch.
To me the choice depends on the recipient of the quilt , the fabricss, and your skill level. I think it would be fun to make a simple split rail with solids or blenders and go all out with the quilting using some of Leah Day's filler patterns (she's been posting them on her blog) on half of the strips and rows of straight lines on the others.