RB20 loses the same .5L the sr20 does...
dude, go with an rb25. six cyl, good displacement, it's toybochahjed, if you blow the rear end up, replace it with a heftier one.
If you're building a track/show car, price should not determine something like "rb20 or rb25". Build what you want. you're not going to spend a sh*t ton more money to go with a 25 rather than a 20, and you'll get more benefit from it. Also, if you go with an rb20 you have to make sure it's a 24 valve, whereas all the rb25's came with.
ooh lah lah, and if you get one made post 93, it'll have that there v-tach (cite: http://en.wikipedia.org/wiki/Nissan_RB_engine )