You are never too old for St Fagans. You need the best part of the day for it, though, so then you'd have less time to do anything else.
I come from Dorset, and have family connections to both Bristol and Cardiff. I think I prefer Cardiff, but it's a close-run thing. I was in Cardiff last weekend and then went into Bristol on the way home.
I would look at comparing accommodation costs, if that is a factor.
Then I would look at the list of things to do and see which my family might prefer. The weather could be a consideration- the camera obscure and suspension bridge will be rubbish if it's wet, and St Fagan's also involves lots of outdoor walking. Cardiff does have lots of Victorian arcades in the shopping area in the centre.
Driving round the centre of either, like most city centres, is a pain, even with sat nav. I think I slightly prefer Cardiff to Bristol for driving.
But if you have never been to Wales, then you should choose Cardiff to rectify that!
Clearly just one night is not going to be enough...