Star and I say this as a fan of both shows who has watched every episode of both shows.
Steven Universe has a serious pacing problem and fails to progress the plot and often falls flat. It has some really good episodes but it also has so many episodes that are utterly pointless wastes of time to watch where nothing happens.
Generally the plot-focused episodes are pretty good while every other episode is mediocre-bad. Problem is 70 percent of the episodes are not plot focused.
Star vs is entertaining even with it's non-plot focused episodes and it has far less of them in comparison to plot-focused episodes. Not to mention that it manages to spread the plot through even many of the non-plot focused episodes, which makes those episodes more interesting. It also avoids the terrible pacing issues SU has.
To be blunt. Steven Universe feels like a 2 season show awkwardly stretched over far more seasons with copious amounts of worthless filler.
Star on the other hand (at least at the moment) feels like it's covered exactly the amount of plot and content that it needed to cover over it's first 2 seasons.