I am a huge fan of both series. As a kid though, LIS rated higher for me than ST. The episodes where Smith was a true villain were the absolute best. Smith started adding the comedic part of his character in order to be employed longer. He realized that a true villain could not last on the show. And for seasons 2 and 3, you can thank Batman for it becoming so campy. Who can forget the Great Vegetable Rebellion?
ST on the other hand, at least for me, maintained the same level of seriousness throughout its run. Yes, some humor was added as mentioned in other posts, but it still remained a serious Sci-Fi series.
Because the two shows were so different, other than being Sci-Fi, it isn't really fair to compare one to another.