Is it better to mount the fans to the rad and then the fans to the case, so the weight of the rads are held by the fan's shrouding?
Alternatively, is it better to mount the fans and rads at the same time, using longer bolts which go thru the case, fans and then into the rads, so that the weight of the rads is held by the case, to a certain extent?
You can do this how ever it works best for you. It doesn't make any difference to the functionallity. Usually this would mean using long bolts because it's easiest this way (there are long bolts included with the radiator bezel that is availible in the shop). However, there are two things you should take care of when using long bolts:
1. Don't fasten the bolts too tight. You can deform the fans shrouding by this which can result in the fan becoming noisy or even blocking the fan.
2. Watch out not to use too long bolts! The threads in the radiator doesn't have an end stop. You can damage the radiator if you use too long bolts.
Also while i'm at it: do the AC rads work as well with the air being sucked thru them as they do with the air being pushed thru them?
Generally, the cooling effect should be the same, but usually you'll want the fans to blow the warm air out of the case. So if you don't have a very fancy case design this fact will dictate the direction of the fans: sucking through the radiator and pushing out of the case.