well, I guess its time for my 2 cents...
From my research I've found that the sr block can definitely take alot more boost on stock internals, and will have much more aftermarket support, than the ca.
But the ca seems in itself to also be a very well constructed motor, with the iron block it seems to be able handle quite alot, although I have heard of the japanese guys having problems with it at high boost levels, with head bolts streching and blowing head gaskets. Maybe having to do with the different materials of the block and the head and thermal expansion? I'm not really sure... as I've never owned a ca powered car... YET!
In my opinion, all nissan motors are very well built and the choice of motor really comes down to personal preference...
-ca18-quick high revs, similar to a honda motor (only in revs, I'm sayin if you like the high revving ability of a honda, you'd probably like the ca)
-sr20-still high revs, dont rev it too high though or your rocker arms will fall off great aftermarket support and help if you ever experience a problem
-ka24-low torque, somewhat good for turbo with ALOT of modifications to the internals of the motor (most definitely pistons, those cast ones arent gonna cut it, maybe some cams to bring that low end to the middle of the powerband). I have seen the stock rods (shotpeened and cryo-treated) handle 400 hp on a different forum.
ok thats my 2 cents