What are your dietary restrictions? If cutting out extra calories is your goal, you can narrow your choice to a stand-alone hot air popper or a microwave popper that doesn't cook with oil. However, if you realistically can't see yourself cutting oil out altogether, there are poppers that will also make flavorful popcorn with just a minuscule amount of fat.
What are your space constraints? Think about whether you want an appliance that will sit on your counter at the ready or if you'd prefer to store a somewhat smaller bowl in a cabinet or closet. Also, take a look at the shape of the popper to ensure that it will stack neatly with other items in your cabinet.
How big is your microwave? If you are considering purchasing a microwave popper, you'll need to take measurements of your microwave's cavity. If your microwave is too compact, certain microwave poppers might not fit inside.
Is the popper compatible with your microwave? Some poppers don't function well with certain microwaves. Worse yet, it may do damage to your microwave. If you are looking to buy a microwave popper, consult your owner's manual or call the microwave manufacturer to check whether the popper is compatible.