I've had both, a ca18det 1200 and a sr20det 1600 (how common!) and both engines are good.

CA's are cheap - i bought mine for $500 with everything but it had a spun bearing. Once rebuild it handled all i gave it with ease. cast iron block and far superior valve actuation in the head make it a great little motor. there are a few things that you should do when rebuilding them and they will give you no worries. They are harder to make big power out of though and aftermarket goodies are harder to get.

SR's are good because they have bags more torque than a ca, aftermarket bits are a dime a dozen - forged pistons, cams etc but they have a bit of an average rocker arm setup and will be limited if chasing massive power by the alloy block but can make an easy 250+ kw on a standard bottom end.

Anyone who hates either in an datsun and only has eyes for the a series has usually not ridden in a datsun powered by them. In saying that, an a series that is built using the money you'd save from buying a later model motor would go fairly nicely.

personally i have love for all mentioned engines