re: Spline material

Use the white oak for the splines. It is good to have a contrasting color and it will match the top insert.. Also, if you plan to put a tray in the box, make the splines  deeper so they stick through and create rests for the tray!
As for splitting or breaking off on the thin pointed ends- any wood can do that if it is not glued in securely and it is cut with a rough cutting saw. Glue the spines in plenty good and saw them with a bit sticking out and sand them flush-- always


cheers, Jim