Rb20's are strong as f**k on standard internals, CHEAP and reliable. You do need to rev them a bit to make power but its not too bad. I still love mine.. Roys car is a good example, good power on a big turbo and not too much lag.
Of course the Rb25 is naturally a better motor (how couldn't be with an extra 500cc?) But they are still fairly expensive($2000+), not to mention if you put one in a R32 you will need to upgrade to a rb25 box (another $1500) as it will destroy the RB20 box with ease. Theres also the hassles with the driveshaft, speedo sensor and sorting out the wiring and gearbox mount.
My opinion - If I had the cash I'd love a RB25.. but I'd rather spend it elsewhere on the car, so I'm happy with my little RB20 which sees limiter daily and doesn't complain! - Don't forget it sounds f**king awesome doing it