Either role is a fantastic opportunity and Depot Driver just means it will take a little longer to get to mainline.
Im gonna guess that my November course will be for a few Cambridge Depot Drivers as well, I'm not sure if mainline would do the same course as Depot, then bolt on any extra bits at the end? The courses must be very similar as both roles end up with an EU licence, although the Depot Drivers licence will have restrictions.
Me personally, I think it'd be better to cut your teeth as a Depot Driver, build your underpinning knowledge and confidence, then it should make the transition to mainline a lot easier. That's just my thoughts.