I have traded in, rebuilt and sold these engines for years. Most of the label B&S or others stick on the engines are just that, Gold, Platinum, I/C, etc.
Now, the big difference comes in the 31 series engines, between 310707 and the 31 engines that have a letter in them such as 31C707. The latter engines use a different crankshaft, different piston, redesigned counter balance system, different sump for example. Better? Well the counter balance system should help with the worn out counter balance bushing problem, otherwise, I don't see any real improvement.
Something else I will throw in here. So many people, even experienced people, are fooled by the filter on the side of B&S engines. Most of these are NOT pressure lubed as we think of it. They have an oil pump, they pick up oil from the sump and circulate it through the filter and most of them then route it to the bottom main bearing only. That way they can say "pressure lubricated". They still use a slinger for all the rest of the lube. Some are actually "FULLY pressure lubricated. Even the 18 hp 31G777 ELS is mostly slinger lubed.