They both connect to the same server, so it simply depends on what you want to use. There are other IRC clients out there as well if you don't want to use mIRC.
Third option: Trillian. Trillian is a messenger client that works with just about everything out there, including IRC, AIM, MSN, Facebook, Google, Twitter, you name it.
https://www.trillian.im/
I really like it, personally. Be worth checking out.
I'm running 4.1 for free at this point. I can't justify the Pro version financially, but this one seems to be working just fine. The iPhone app costs $5, but the desktop free version is just fine.