Determine what type Mod you want, then determine the best Mod in that catagory for your vaping needs. One thing to consider, is the accessories required to make for a great Vape. An Empire for instance comes with a drip shield, and operates with an IMR 18650 battery, or a smaller IMR battery to give you options on size..however it is a native 901 connection, but a 901 to 510 adapter is available. Other mods will still require buying a drip shield, or a DCA which uses 306 atomizers.
Your choices are many and varied, so first decide what type, then what kind, then what accessories are needed, Then Buy It.