The CA18 is a great driver, but the ER18 will offer a bit more upper midrange presence.
The CA18 is one of the easiest drivers in the world to work with. With no breakup node, you can mount these in your doors, provide a high pass in the 60-70 hz range and call it a day. You can basically run them full range with no ill effects. That makes them suitable to match with a number of tweeters out there, though I still suggest the Seas Neo. You could almost do a simple passive network for the tweeter (12db/octave slope at about 2800 hz) if you're willing to have some patience and try some various crossover points out. Though active is still the much easier way to go.
Either driver is a fine driver. The CA18RNX is well worth the money. The midbass performance will astound you.
Oh, this is all assuming that your doors will be properly treated.