Completely different routes and traction - differences that'll only become even more apparent as the South Wales metro gets introduced.
As a new driver at Cardiff mainline, you'll mostly be driving to the likes of Ebbw Vale, Maesteg and Bridgend via Barry, plus I think there's some work to Swansea and Cheltenham too. Quite a few station stops in those jobs, but some higher speed running too, with the chance to drive at 95mph often and occasionally the full 100mph. You would be driving class 150,153,158,170 and 175 units.
As the years progress you'll get the chance to learn longer routes, to Carmarthen and eventually Milford Haven, and all the way up in the other direction to Crewe and Chester. If you stayed long enough you might even get the chance to drive the Premier service with its dedicated Class 67 locomotive and MKIV coaching stock.
On the Valleys you'd be driving class 150 153 and 769 on the Valleys network, which is basically everything that passes through Queen Street station (with the exception of services to Bridgend via Barry which as mentioned above are driven by mainline drivers west of Cardiff). Stopping at almost every station all day long, it can be very intensive work with little time between stations, but some people prefer it. In the future they'll be driving the tram trains, complete with a short section of street running in Cardiff Bay which will certainly be a change.