a stock HS50 (or HSSK50) is definitely more powerful than a HS40 (or HSSK40). And frankly, due to emissions, at some point the HSSK40 actually decreased in CCs (so it's more like an H35), but Tecumseh still calls it a "4 hp" engine.
The amount of work to rebuild a HS40 is exactly the same as a HS50. So why use an HS40? .... well unless you have a pre-1973 HS40 with a rounded blower housing (like say on a Rupp), and you want to maintain that "look." And HSSK50 rings sets are generally easier to get than any HS40/HSSK40 ring set.
And let's be frank here... we're all old. And probably fat. well certainly fatter than the 12 year old version of ourselves. so don't you want a little more power that an HS50 would offer? of course you do...