I think the 1215C is the largest 1200 series CPU recommended for most applications, and has both DC and relay output versions available. With the other 1200 CPU's (1211, 1212, 1214, 1215), the amount of memory and IO expansion scales up with price. They all have the same three power options (AC/DC/Relay, DC/DC/DC, DC/DC/RLY), have very similar onboard IO layouts, and are programmed the same.
The 1217C is a little different. It has some 24V DC IO, but it also has some Line driver IO, which is a very high speed (1MHz) differential setup at a differnet voltage (2.5V maybe?). I've never seen an application that called for it; I'm told it was developed for a very specific user segment. Unless you specifically want Line Driver IO, generally you'll want to stick with something in the range of 1211-1215, and leave the 1217 alone.
Hope that helps!