When to use a spline vrs a biscuit joiner?
If you've got a biscuit joiner, then I'd just use that. They're 10x easier than splines, and I'm not convinced that there's any advantage to splines.

There may well be exceptions, depending on your exact circumstances, but since you didn't list any specific circumstances, we don't have anything else to go on.

Something I built with splines: http://www.instructables.com/id/Expandable-formal-dining-table-that-seats-ten-and-/ (see step 12)

Something I built with biscuits: http://www.instructables.com/id/Pantry-Cabinet/

Frankly, I haven't done a spline since I got my biscuit joiner.