Edge miter joints offer the ability to completely hide the end grain of the joined pieces, providing a smooth, clean surface to view. The largest problem with miter joints is that they are very difficult to assemble. There are, however, some variations that offer some assistance in assembly.
Edge miter joints actually expose more wood surface, and therefore the glued joint produced can actually be stronger than a glued butt joint. If more strength is required, you can use small glue blocks, or a continuous glue block. Cut these blocks so the grain runs in the same direction as the pieces they are going to join, so that the two boards and the block expand and contract identically.
Using some sort of mechanical fastener such as screws or nails can hold edge mitered joints together, however this can ruin the esthetics of a piece, unless the fastener can be painted over or camouflaged.
Using biscuits in combination with an edge miter joint is an ideal way to have an easy assembly process while having the beauty of nearly-invisible miter joints. The biscuits don't make the joint stronger (in fact, they can weaken it), however this is a small price to pay for side-stepping the frustration of assembling edge miter joints.
When the joint is assembled, the fit of the biscuits will determine how well the joint keeps its shape during glue-up. By choosing a biscuit that has a snug fit into the biscuit slot, you reduce the sideways movement that can lead to misalignment under clamping pressure. You can improve this by placing the biscuits even closer, as close as 6 inches if required.
A full-length spline is a convenient substitute for a biscuit jointer, and can even be more accurate. A hardwood spline is ideal, but if you intend to use a hardboard spline, measure its thickness before making any cuts: hardboard is often narrower than its stated dimension. Cut the slot into both pieces of wood no more than 1/3 in from the inside corner. Cutting it any closer to the point will leave very little wood intact to give mechanical strength to the joint.
Remember, a spline will not increase the strength of the joint, however it can make the joint weaker. Its primary purpose is to provide positive registration for the assembly and clamping process