Options include color, type of pieces (patio or select), thickness (¾” up to 4”), and texture (smooth, rippled, clefted, tumbled). You should also consider the application of the rock (pool, patio, walkway) and water exposure (quartz vs. sandstone).
Patio indicates smaller diameter “stepping stone” type pieces while Select or Stand-Up indicates larger pieces. Select pieces are generally more expensive because they are harder to produce. It’s more difficult to split a large piece of stone without breaking it into smaller pieces.
Use the thinner flagstone when you have an existing subsurface like a concrete floor or a CMU block wall. Use thicker pieces when you dry-set or sand-set flagstone over dirt.