Which is better experience for becoming an officer, the UOTC or enlisting as a reserve soldier?

If you are still at University, I would suggest that you join the UOTC.
You will receive instruction from Regular soldiers on what everyone else in the Army is getting instructed on.

If you are not at University, they join the Reserves.
The Reserves only parade on one or two evenings a week and carry out a two week summer camp every year.
Don't forget that to a lot of people, the Reserves are a "hobby" not a job.

With either option, you are able to join the Regular Army from UOTC or the Reserves as you will have to attend RMA Sandhurst to carry out your Officer training, then Brecon if you want to be a Infantry Officer.

I hope this helps you make the right decision.