Ed's post invites the contrary view, naturally, one interestingly shared by the Golf World rankings panel back in the noughties, when the latter was still a reasonably serious exercise: RStD was consistently placed above RPGC, in part, but only, in part for reasons of conditioning. A disclaimer - I am lucky enough to belong to both Porthcawl and Harlech, and find it really hard to separate them (not least as they are so different) although I can certainly see why many people seem to prefer the former. Prolonged acquaintance with Harlech does, however, make you realise just how good and strategically challenging are some of the unfashionable, flatter holes going out (especially off the white, medal tees): the 3rd, 4th, 6th and 7th are all really strong golf holes, made moreso by the prevailing wind.
If you play the old game of a match between the two, then I would suggest the following outcomes (with which, again, others will disagree)
1st RPGC
2nd RPGC
3rd RStD
4th Halved
5th RPGC
6th RStD
7th RStD
8th RPGC
9th RPGC
10th RStD
11th RPGC
12th Halved
13th Halved
14th Halved
15th RStD
16th RStD
17th RStD
18th RPGC
which happily comes out at all square. And given that I have many very good friends at both venerable institutions, I am more than content, personally, to leave it there.