Warped though it may seem, I tackled this very thing with my first project; 4 L shaped + 5 U shaped (cropped to contour an alcove), with biscuited mitred corners; material was cheap an tres nasty ply edged with mahogany. 3 years on they're still as rough as they ever were, the ony changes being they've darkened with age.
If I were to do it over in solid wood however, I can't say I fancy either option; loking at long grain on one side, with end grain on the other strikes me as being just.....wrong, although Alberto has a valid piont re keeping the grain flowing in the one direction. Even with the pieces of each shelf cut from the same board, a continuously glued mitred joint may fail through slightly unequal rates of expansion / contraction.