I think it comes down to personal preferences. rcirc is a single file. ERC is more modular, it has the core in one file and a bunch of modules can be pulled in for extra functionality.
I originally went with rcirc because I preferred the way it handles logging: it maintains a separate log buffer and you can format the local logs separate from the display format. ERC logging works by writing out the recent portions the buffer itself. Both are very customizable though, lots of variables and hooks so it should be possible to tweak either one to your taste.
