mIRC uses IRC (Internet Relay Chat). Although that is still a way to chat, it is NOT an instant messenger at all. IRC was available long before Instant Messengers. IRC is a LOT of networks, each with a lot of servers, where the biggest networks have over 100,000 different chat rooms. Chat rooms can sometimes have thousands of people in them at once. It's not just a one-on-one way to talk to people. The emphasis is more on community than on individuals.
IM is good for quick conversations between people you know. IRC is good for long conversations, or other things such as trivia and such, with people you don't know yet (and those you do know). You don't meet people on IM very often... it's usually someone you know or else someone who knows someone you know. On IRC, you meet new people all the time.
It is up to you what exactly you're looking for. They really can't be compared to each other. Both have their uses and both have pluses and minuses. I use IM for people I know to quickly contact me. I use IRC to chat with lots of people, play trivia, script, etc. For me, IRC is much more important to me than IM. For others, it's the other way. Some will support IRC and some will support IM and some will support both. Only you can really decide what is best for your needs.
As far as security, it depends what you choose to do with it. You can use SSL connections if you choose to, so that is secure. It's pretty much secure regardless if you use SSL, though.
The biggest thing that can make it less secure is when you run scripts or commands that you know nothing about. I'll warn you right out not to ever type a command that uses $decode because those are usually dangerous in one way or another. As soon as you type a command or run a script that you don't trust and/or know something about, you risk having problems. With MSN, you don't have scripting, so this isn't an issue. In that way, you *could* say MSN is more secure, but you don't *have* to run scripts or commands you don't know, so the amount of security in mIRC is up to you.
Also, remember that downloading anything online, from IRC or IM or FTP or wherever, you risk getting a LOT of viruses.
Finally, mIRC isn't meant to be fancy, or as you worded it, it doesn't have a lot of frills. At least, not plain "vanilla" mIRC without any script running. You can get scripts that can make it into something really fancy and with a lot of frills, if you really want to. mIRC isn't quite as intuitive as IM (it's not as easy to learn), but if you are willing to put in a little effort, it isn't really that hard, either. I definitely recommend it. It can be a lot of fun, and if you feel like learning to script, you can make some really cool things.
If you do check it out and you decide that you want to try out some scripts, I would recommend a source like http://www.mircscripts.org/
because they check all scripts that are submitted for backdoors, keyloggers, etc., so you shouldn't have to worry too much about those ones. You can also ask us here about a script to see if we have opinions on it.