Just keep in mind that portable is not exactly the same as having the settings in your install folder. Now, unless -portable is different than installing it as portable from the installer, which I don't think it is, then it disables some things that you might not want disabled. For example, if it's being used in portable mode (and hasn't ever been used in the normal mode), irc:// links won't be registed and so you can't click them in your browser to open them in mIRC. This is because portable mode does not make changes to your registry. Now, -portable is done after installation, so if you actually installed mIRC, this shouldn't be an issue. But if you just extract mIRC without installation, it can be.
By putting mIRC.ini in your install folder, mIRC will work exactly as a normal install, but your settings will all be stored in one folder. That makes it "portable" in that you can easily compress it, back it up, or whatever without having to manage multiple directories. But it does not prevent it from making changes to your registry. In most cases, if it's on your own system, I would recommend placing the ini file in your install folder instead of using -portable. If you're on someone else's or a public computer or something, then -portable is the better option. Of course, you can use either option depending on your preferences. I have 5 or 6 installs on my computer (and that's 3 or 4 different versions of mIRC) with the ini files in the install folder and they all load different scripts and have different settings and there is never a conflict.