Best officers are those that serve in the ranks first.
Also, it seems to be an excellent route to the beige list and more senior ranks. That's just in my experience though from meeting a Brigadier and several half Colonels with rank experience.
The more realistic explanation is most officers would have a combination of getting bullied in the ranks + not enjoying it. UOTC is a protected environment to develop. Plus, the UOTC serves an advertising and recruitment purpose to showcase the Army without any of the nasty bits (E.g. Being deployable and headed to a warzone.)
There's no better way to lead a soldier than by being one first. It will also give you greater respect from day one from those you serve.
In short: UOTC is better for getting into Sandhurst. Being in the ranks will make you a better officer in the long term, from both a respect and promotion perspective.
UOTC is a part of the Reserves, by the way.
SS