Some of the main aspects to consider when purchasing a vape tanks are:
E-liquid Capacity - how much vape juice the tank holds.
Coil resistance (Higher resistance coils provide more of a cigarette like throat hit while lower resistance coils are geared toward producing more vapor).
E-liquid Refilling System - how the tank is refilled with vape juice. Some tanks are top-fill designs, others are bottom-fill designs. In addition, many tanks have additional child-proofing or leak-protection features.
Adjustable Airflow Control - many tanks allow you to tweak the airflow to customize your experience.
Construction materials - consider upgrading to Pyrex glass or reinforced designs to keep the tank from cracking.