Downside to CAI-If you dont get a bypass valve, or your an idiot, you cold get hydrolock=new engine.

Both are easy to install, if you don't buy some major brand name they can be cheap, but at most, it might make your car sound a bit different with mybe three to four whp gains at most. AFE offers kits (I dunno if they have one for and RSX-S) that claim upwards of 20hp gains...

The difference? Most short ram intakes keep the intake in the engine bay. If you want to do this, I recomend not spending more than the cost of a K&N drop in, then cut the air box so that the K&N is exposed. A short ram, for the most part, is just an open element intake.

A cold air intake removes the filter from the engine bay (usually to near the bumper), so that it gets a better flow of cooler air from outside the engine bay. Cooler air is denser, so you can take in more air...

In reality, you won't notice any real difference with either unless you get some software along with it to deal with an increased air flow.