joinerywoodworking
I know how to do either, but want to know if one is better than the other for particular joints (if so, what cases).
Or is it just a trade off between time / labor (spline) and ease / cost (biscuit joiner)?
In my current case I'm attaching a 3/4 inch lip/edge to a glued up table top (for the top I used leftover unfinished flooring, so that was spline-like).
Dowels are probably your best bet for this fix because the tool cost is much lower for the case that you need to purchase tools to do this repair. The cost of a doweling jig is much lower than a biscuit cutter. Drilling the dowel holes is easily accomplished with an electric drill.
If you end up with just a one inch wide repair strip added in then I would just drill the dowel holes right through the strip and into the original material on each side. Do check your table top pieces for overall flatness after you separate the two sides from the split. Warped lumber often develops a curvature that spans a greater distance than just one inch. You'll need to take this into account when you decide to start trimming away the split edges.
With properly jointed surfaces on the two trimmed edges and the hardwood accent piece the dowels and glue should be plenty strong to hold things together.
You may want to cut the accent piece just a small bit longer than the amount needed and then trim and sand it to the existing round table edges after the gluing is done. This will give you the greatest opportunity blend the edge of the table to the nicest fit and look.
I see some challenge in clamping the round table to get good tight glue joints. You may need to fabricate some jigs to help provide a place to engage your clamps across the width of the table. The sketch below shows one idea of how to do this. Some flat boards about 4 -> 5 inches wide can have some blocks glued and screwed on the ends that will provide a place to engage the clamps.
I have shown the blocks somewhat offset from where they would actually be attached to the boards so that you can see the nature of how they would fit to the contour of the table edge. Long bar clamps or pipe clamps would be spanned across the table top at the overhang of these boards as shown in the next picture.
A purest would say to use a mallet made of wood or rolled hardened rawhide, but not rubber. I have a couple of sets and even my expensive ones have held up well using a regular 16 oz hammer.
But remember my motto: Every tool in my box is a hammer, except for the screwdrivers which are also chisels"
Best Answer
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.