If you are buying a stripped lower its neither one. At least here in WI its an other. A typical rifle lower will have what most people recognize as a stock, either adjustable, or fixed. A pistol will typically have what is termed a pistol brace instead of a stock.
In picture above. Top is SBR with stock. 12 1/2" barrel with flash can.
Bottom is pistol with SBA3 pistol brace. 8" barrel with 10" suppressor.
In this picture the pistol is actually longer than the rifle.