I would generally rather have the framerate locked.
If an locked game consistently runs between 30fps and 40fps and never drops below 30fps then IMO the experience is better with a rock solid 30fps than having low processing demand segments running at 40fps.
But if a game runs between 40 and 60fps and never (or rarely) drops into the 30s then an uncapped fps may make for a marginally better experience than 30fps locked. but only if the game spends the majority of the time in the 50+fps range. If it hangs around mostly in the 40-50 range and only occasionally hits 60 then might as well drop it to 30 locked.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
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