I would use the fastest device of both, with most memory.
I personally use my Synology, as it has 2 gigs of ram.
My raspberry pi is still the first version (the oldest one) and it is way less smooth.
The newer raspberry pi’s are both fine.
Another reason I prefer the synology, is that I then have only 1 device that needs to be powered on, as my nas is always on. And I also have only 1 device to update and maintain.
Depends a bit on what you prefer, but i would go for speed first.