Minecraft wiki entry for Clock, associated text: Using two 7-multipliers (×49) is slightly more expensive, but shorter, than getting ×50 with 5×5×2, or getting ×48 with 3×4×4 or 6×8;.
An 8-multiplier is slightly more expensive, but shorter, than separate 2- and 4-multipliers. However, two of them are both longer and more expensive than three 4-multipliers.