Personally, I really like having a clicker with a "button" to push, rather than just a standard box clicker. They are easier to click, and it's easier to tell whether you are holding it correctly (box clickers can get turned around in your hand and when you go to click, you push the wrong end and nothing happens). They do break more easily though, so if you tend to drop things like I do, buy a few so you have a spare.
The i-click is nice if your dog is at all sound sensitive, because it is a lot quieter than a box clicker, but you can make a box clicker very quiet with a piece of "fun-tac" (the goo you can stick posters on the wall with). The i-click is also easier to push than a box clicker, so if you have any sort of hand/finger issues, it can be a life-saver. (A lot of my older students have arthritis and have a hard time with standard box clickers.)
You can also click the i-click with your foot or your knees if you want to click, but are working on something that requires two hands.
Another thing to consider is whether the clicker has a tab for a key ring or wrist band. I like tabbed clickers because I can attach them to myself more easily.
Here's another buttoned, tabbed clicker I like - http://www.entirelypets.com/tcclicker.html
There's also an electronic clicker called the clicker+ that makes several different tones. You can use one for each pet if you have a multipet house. Though that's not really necessary. Dogs sort out whose click is whose pretty easily, in my experience. My agility instructor used the clicker+ to "tag" people when we were practicing our handling skills. And it has a finger strap so you don't have to hold onto it.
It looks like the clicker+ is unavailable right now though - http://clickerpets.stores.yahoo.net/newclicker.html