Depending on what you are looking for, there are many things that you should consider when buying a wall clock. Chief amongst these is the size. That is probably the first thing that I would advise people figuring out: that and the budget which you are looking to spend. This will quickly (and rightly) narrow down a lot of the possibilities.
Take a look at the size of the space where you are looking to put these wall clocks. If it is a very big space, then you can consider going with an oversized one. These are very bold and stylistic, but they won't work in a more traditional or humble setting. Any room in which they are hung will have them as their centerpiece. That's just the way it is.
Much more commonly, people are dealing with a smaller space, such as above a kitchen cabinet or between the TV and a door in the family room. What I would do here is to measure the width of the maximum space that is available. Also measure the height if that may come into play. This will start to get you thinking about size, as well as having the practical value of having an actual recording of the feet and inches available for the wall clock.
From this new number, now you can surmise what you would like the size of your ideal wall clock to be. Maybe it will be 10 inches in diameter or maybe it will be a big 14 inch diameter clock. I would try to avoid a situation where the clock is partially blocked from view by a protruding piece of furniture, but this can't always be prevented.
Now let's take a little look at budget and how that plays into wall clocks. First of all, on the low end it should be very obvious to anyone who has ever bought a clock at all that these things can be had on the cheap. It is not uncommon to see stores trying to offload their store of wall clocks for $5 each. Of course, these are not artistic masterpieces by any stretch of the imagination. But we can see from this that the lowest price is very close to zero. That's good, because it means that the wall clock that we eventually want will probably be that much closer to zero as well!
I will just say this: you have to be willing to spend a little money in order to buy wall clocks that are solid and substantial and will withstand the test of time. For one person, that may be $20, and for another that may be $50. It all depends on what sort of a material you want and how high a level of craftsmanship and quality. Remember: a wall clock is a very important and integral part of any room in which it hangs. It is kind of a secondary focal point. And beyond this it adds a lot of subtle flavor to the room. That is why I am so diligent about telling people to really take their time and see what's available before settling.