If you are buying either of those DACs, a USB connection is the easiest way to get best quality sound. With the RPi 4, there is no benefit to using an S/PDIF hat - you only need an S/PDIF hat if your USB port introduces jitter (a problem with the 3B+) - spending money on a a HAT instead of using the an RPi 4 seems to make things more complicated without an obvious benefit.
So, I would recommend getting the RPi 4 and using USB to the Topping or Modi. Topping’s D30 seems to test really well, and they seem to have less issues than Schiit - though I think some people like the sound from the Schiit DACs/
I have read some mixed reviews regarding the USB port noise on RPi 4. Some people say that the problem was resolved, but there is also this guy: youtu.be/cjqEPyMr1zI
It would be ideal for avoid any HATs… are there any detailed comparisons of RPi 3 vs RPi4?