Really depends what your goals are. How partial are you to your RB20?
Usually in these circumstances it's easier to drop an rb25 in. By keeping the Rb20 you'll always be behind the '8 ball', given that the extra 500cc is a 25% increase over the stock rb20.
Depending on your power goals, I'd personally take a rb25 if you can do it cheap enough.
Edit: was also going to say I hate mods going to waste. But a lot of yours could be applicable to an rb25 set-up. Now is as good a time as any to make this decision. Before you spend money for specific rb20 components
so I'm guessing you're just about to finish school?
You need to keep in mind what type of car you're looking at and how much it will cost, including rego, insurance, petrol and general maintenance. Skylines aren't a cheap toy, young padawan.