Thee are a number of threads about this and folk here have quite strong views on both which will come out soon i'm sure.
A strong point about the OTC is that it is normally based around your studies and university year so your training happens in term time or, hopefully right at the start of holidays. If you go home in the summer it may be difficult to attend TA training 200 miles away.
You will also hopefully get the opportunity to try or look at all the different arms and services and get attachments to some. While you may have "decided" which area of the army you want to go in I've seen this change once students have become more aware.
With both, what you get out of it depends what you put in.
If you want to PM me with which OTC you are considering joining I may be able to give you more specific info.