FWI, over the past several months I've tried Seafoam in every tank (1 oz per gallon) then STP (3/4 oz per gallon) in pure gas by the recommended amounts per product.
For me, I've noticed the engine ran smoother and consistently got better fuel mileage with the Seafoam. Prior to Seafoam, I avg'd 40 mpg and with seafoam got a high of 45 mpg. The best I got with STP was 42 mpg.
The primary reason I use an additive is the unbranded gas our bulk plant sells has no additives in it like Techron and a tank of gas may last two weeks. I had read a post a while back where the author suggested he got better fuel mileage with Seafoam and after running both products, I would have to agree.
Maybe there's a better benefit to STP vs Seafoam, but from an operator's stand point I'm not seeing it.