I think the historical use explains the difference between the HT and SC. The former were used as infantry carriers while the latter was a cavalry/scout vehicle used earlier in the war. The difference may seem minor but a scout unit would be a bit less doctrinaire with regards to weapons deployment.
A US Cavalry Troop in 1942 had a mix of jeeps and M3a1 SC (4 and 1 per section respectively, but only 2 sections per platoon). A section had 19 men, 2 .30 cal 1 .50 cal and a 37mm AT gun.
A platoon had two sections - 42 men, 8 jeeps, 2 SC, 4 .30 cal, 2 .50 cal, 2 AT guns (then add two more jeeps, another .30 cal and an 81* mortar in a mortar section)
A cavalry troop had 3 platoons plus a troop headquarters with 55 men, 2 x 2.5 ton trucks for cargo/baggage and a kitchen, a plethora of 3/4 ton trucks (8), 3 more jeeps, 3 more SC and a whopping 6 .50 cal (various mounts).
The late 43 version was simplified to 6 jeeps, 3 60* Mtr, 3 .30 cal, and 3 M8 AC in each of 3 platoons plus the Troop HQ with its mass of equipment from jeeps to HTs and ACs along with nearly 60 men.