I had the same dilemma as OP, but...
- Cupra ST looks amazingly better than the Golf R estate (the Golf rear is... boxy)
- The Cupra had many more toys standard than the Golf, so... better specs.
- I think (cannot say for sure) that the Cupra might be marginally more spacious than the Golf.
Yes, the Golf R does sound better, actually. MUCH better but... The Cupra is a head-turner and the Golf, even in R form is a bit "meh!"
I did
shop for both and found way better deals for the Cupra - I got a fully loaded Cupra, almost new for less than I could find an entry level Golf R - the Golf had no DCC (pretty essential), no tech packs and no upgraded Sound system - get Seat Sound System over Beats Audio, since it is an overall better system.