I'd recommend you buy half a dozen and keep them in every coat pocket and treat bag, that way you never have to remember to take one with you. I prefer this style of clicker myself, mainly because it sits nicely in the palm of your hand, but almost any will do. There is one particular one called the multi-click, I think, that has different levels of click, which is good for dogs that are sound sensitive. The only ones I really don't like are the rectangular and bone shaped ones, as I find them uncomfortable and awkward to hold.
Clickers are far more consistent and precise than any other method, as they capture and mark the exact second your dog does something right. They also get the dog to think for themselves, as the dog can easily identify precisely when the click happened and decide to repeat that behaviour to see if they get another reward.
You can use clicker not only to mark specific behaviour, such as sit, down etc, but also to shape a dog's behaviour and in doing so your dog gets a fantastic mental work out as they have to problem solve along the way.
I'd also highly recommend Kikopup and Karen Pryor is a good place to read about why the clicker works and therefore get to grips with the best way to use it.