I'd agree with Gaz, Newport would be the better option as you will tend to get a bit more freight there...........the workings to and from Pengam and Wentloog being the extras.
As for what you'll see at Cardiff, everything will be DBS 66s bar the Murcos which will be in the hands of DBS 60s. If any are spare, a 60 may turn up on steel.
The logs from Baglan Bay have recently gone over to Colas 60s though a 66 was on it last week for the first time in a very long time.
Depending on the day, you may get a GBRf 66 on the stone from Neath to Pengam and you will also get the DCR 56 coming from Pengam to stable by the station.
You could be lucky and get a Colas l/e move from Canton to Westbury ehich may bring you a 70.
You'd get more GBRf workings in through Newport.