Using IRC chat is a big deal for Mac and iPhone developers. It's a great way to find peer-to-peer support. Sites like Freenode and OSX86.hu give you access to hundreds of other developers to consult with in real time. Engineers from a major fruit-named corporation frequent popular chat rooms, offering unparalleled technical access. Having a useful IRC client on the go can be golden, so it was important to me to find a good one for the iPad, and both of these clients are excellent.
When it comes to price, Colloquy is the clear winner. The iPad version of Colloquy sells for US $1.99 at the App Store. Linkinus costs about four times more at $7.99 for its iPad version. In terms of bang for your buck, both are good buys, though. I don't think you'll be dissatisfied with either one.
If price is not the main issue, then it probably comes down to ease of use versus control. Colloquy provides a clean, simple interface with nice user-centered features like nickname completion. You can find chat room participants just by typing a few letters. Linkinus, in contrast, offers far more technical feedback during use; that's the stuff that Colloquy filters out in favor of less cluttered presentations. If you're looking for simplicity, you cannot beat Colloquy, but if you're looking for more technical control, then Linkinus may be the choice for you.