Start with the size - this is the most important feature you'll need to compare in order to ensure you're spending your money on something viable. Party tents come in all shapes and sizes, and if you've got more money to spend you can even make a customized order that really suits your needs perfectly. As long as you're browsing through generic models though, make sure you know how big you need your tent to be - not just in terms of sheer space, but also in its capacity for people, as sometimes sizes can be misleading.
Well-designed party tents tend to be very flexible in terms of size though - sometimes you may not need to use the whole tent when you're throwing a smaller party, in which case you'd want to have a party tent that can be adjusted during its construction, which is typically done with a modular design that allows you to add and remove components on the fly. This isn't just useful in terms of dynamically adjusting the tent for different numbers of people, but it can also be quite convenient when you need the most portability you can get, as party tents designed in this way tend to be very easy to pack into a small space for transportation.
Of course, don't just look at the size when you're comparing party tents, and make sure that all the features of the tent you're buying are suitable for your specific needs. You may need extra durability and longevity - or on the other hand, better resistance against rain and, on the other hand, sunlight - all of these are features offered by different designers of party tents and they typically have separate models covering all those bases, so all you have to do is make a list of your requirements and go shopping with that list in hand.
It's not a bad idea to ensure that the manufacturer of party tents you're using also has flexible and well-designed warranty policies covering their products - there's hardly anything worse than having to pay a great deal of money shortly after having bought your tent, just because it was built in a careless manner and it incurred some serious damage which you can't repair on your own - and when it's not covered by your warranty either, you have a real problem on your hands.