It is a matter of personal preference. If you ask people which they prefer, you will expect mixed answers.
However, I like how bottom-loading dispensers work more than top-loading coolers.
You can simply shimmy in the water bottles at the bottom without lifting those heavy jugs like what you normally do with top-loading coolers.
Again, whether it's bottom or top-loading, you should buy a water cooler based on what you need and its convenience, regardless of type.