Just go with the RB25, it will have more than enough power and revs to put a smile on your face...right out of the box! If there were a lot of benefits to doing this swap Im sure it would be a hot topic here and the Skyline forums.....but its not.
I run a RB20 in my S30 because ultimate power wasnt that important, if it is to you then the RB25 is a better platform to start with. And dont forget, your Z31 is a fair bit heavier than my S30 so go with the setup that offers more torque!
The RB25 tranny is better, stronger and typicaly worth a lot more! Overall there isnt much that you would want to keep off the RB20 and use in place of RB25 parts.....nothing. There may be a few spares, ignitor...coil packs..... Injectors are not compatible.
And on a selfish note, ....I may be interested in a spare RB20! Cambridge isnt that far and I need more clutter in my garage! Just pm if you interested.