Both of these are huge areas, which offer a wide range of options!
You could live in a city in both locations, or as rural as you like. You could go coastal or inland in both. You could go touristy or 'off the beaten track'.
Devon has coast/moors/rolling farmland, whereas s Wales has coast, hills & valleys. There's also pockets of big industry along the Wales coast, which mean it's not quite as scenic all the way along.
A big factor for me would be how much you want to be accessible to other parts of the UK. In S Wales, once you get to the end of the M4, the roads are slow, and getting anywhere takes ages. In Devon you do at least have some major A roads once you fall off the bottom of the M5. The traffic in Devon in the summer is terrible, because it's the only way to get to Cornwall.
There's a lot to be said in both locations for living close-ish to where tourists want to go. There's likely to be more going on in terms of pubs/restaurants and 'culture' in general, and also potential holiday jobs for teenage kids.
They are 2 of my favourite places in the country - I grew up in Devon, and would love to live in S Wales now (DH having none of it, we also live in the midlands).
If you have family in Birmingham, have you considered Bristol? Very easy to access the coast, but also very easy to connect with home?