Wait for the Xbone x tbh. Pro support for 1080p is laughable.
While I've already decided not to bother with a PS4 Pro, nowhere in my OP did I say I was considering an Xbox One of any variety. I have a good PC for playing games on, so if I want any MS exclusive games then I'd get them on that. The Xbox One X would literally be a £450 paperweight, and this is the reason I sold my original Xbox One. The best use case I can think of for that system is BC, at which point I could save £350-odd and just get an old 360S.
At any rate, after reading some replies about how the difference isn't big enough to justify it, the only thing really selling me on PS4 Pro is the 1080p Remote Play, but to be honest I have plenty of games on my PC that I can stream to the other room, plus I have a Switch too, so I'm not exactly gasping for the feature.
I was hoping PS4 Pro would be better for 1080p sets. Downsampling at a system level seems like a must-have, as would more consistent offerings in terms of performance settings from developers. That Sony is even dropping the ball on that front tells me all I need to know: 1080p is an afterthought for this system.
In short, doesn't seem like it'd be worth the faff of changing the HDDs, packing up my Slim, selling it and such.