So at this point, I have been fortunate to have travelled on the majority of the larger South-West heritage railways (except the Bodmin and Wenford).
South Devon railway:
Enthusiasts' perspective: If you are interested in what a quintessential GWR branch line heritage railway looks like, this to me was the best at recreating it in terms of GWR heritage railways. Plenty of classic GWR tank locomotives and GWR rolling stock around the line, and it has in recent years been practising its niche well as a majority tank engine line. The museums the line has are rather good and I found the volunteers and members of staff there very friendly.
From a non enthusiasts perspective: The line has plenty to offer scenery wise, the scenery on offer is great along the River Dart, the train journey is in what I like to describe as a nice distance of 7 miles, so the train journey for non enthusiasts doesn't outstay its welcome but isn't too short for them to go 'is that it?!'. More importantly, it's a heritage railway where you can definitely make a day out of it. If you buy a 3 one ticket, you can gain access to the Rare Breeds centre at Totnes and the Butterfly centre at Buckfastleigh. Totnes itself is a stunning medieval town to look around, with a good mix of independent and appropriate chain shops in the town centre.
Paignton and Dartmouth Steam Railway:
Enthusiast's perspective: The line is excellent from an enthusiast's perspective, there is a decent gradient from Kingswear up to Paignton and there is plenty in the way of infrastructure cameos from tunnels to viaducts along the course of the line. The locomotive fleet is fairly decent, a mix of GWR and BR Standard locomotives, with rolling stock mostly consisting of MK1s.
The Scenery is another standout of this line. The views of this line are perfect along the line, such as following the coastline from leaving Paignton to then following the River Dart towards Kingswear, which is another good approach. Going back to the gradient, if you want the best of locomotive chuff, you won't be disappointed. I was staying across the bay in Brixham, a good few miles away as the crow flies, and you could hear the locomotives from the end of the harbour as they approached the harbour front.
Non-enthusiast's perspective: The scenery will be the main selling point; it is stunning along the course of this line. I cannot comment much on Kingswear/Dartmouth, and I will explain further on why, but as for Paignton, you aren't short of things to do, there are plenty of shops and places to eat within the town, and the beachfront is decent as well.
My problems with the Paignton and Dartmouth: The Paignton and Dartmouth is, on paper, a really decent railway, but in execution, it is flawed by its operating model. This railway is not a conventional railway in the fact that it is more of a privately operated railway than a typical heritage railway. I am not a fan of its railtour style approach as opposed to a regular heritage railway model. I wasn't exactly happy to be pressured into having my ticket be for an allocated train and seat only as opposed to a heritage railway where I could explore the line at my own pace.
This would be understandable if it were a peak Saturday in August, but this was a Thursday afternoon in Early July, the train wasn't exactly busy on either journey and if I am confident that if I got a return a few hours later, I would find a seat fairly easily. So basically, I got a general round journey for the next train back, bought a latte (was pretty decent to be fair) and admired the scenery at the harbour wall at Kingswear, taking in the view of the Dart valley. I unfortunately didn't get to go round Kingswear proper, so I can't comment on the town, but it certainly looked scenic.
I wasn't overwild in how stripped back the line felt. Churston on any other heritage railway would have been decorated in historical adverts, maps and signage; instead, the station looked rather sterile, and you weren't allowed off the train to stretch your legs or to film the train pass on the other platform.
So, in conclusion, if you haven't done either and you want to do a heritage railway with your family, I would highly recommend the South Devon railway, it is my favourite heritage railway in the whole of the Southwest of England. However, if you decided to return to that area of Devon, I wouldn't write off the Paignton and Dartmouth entirely if you know what to expect, the gradient, loco collection and views are great, but I would recommend this line more to an enthusiast who would want to enjoy those aspects instead in one round train trip.