Daveman has given you the obvious difference.

The RB20 is a favoured engine amongst the drifters as it is very suited to high revs for long periods.

The SR20 is basically one of the strongest well built 4 cyl ever made (my opinion)

The CA18 is another good engine that nissan has produced, favoured in transpants because of it smaller size and easier to certify (engineer) in most states.

The CA18 is a more than cabable engine i am doing this conversion in my ute. Engineering was the decider for me but looking at the big picture you don't need alot of power to push sch light cars down the road. Power to weight is what most people look at, but really I believe it should be USEABLE power to weight. no point having heaps of power if you cannot put it down to the ground.

I wouldn't be detered from a CA18 because its a 1.8lt not a 2lt. drop in some 40thou piston and increase the capacity to close to a 2lt anyway