Hi and welcome to the forums! I'm not familiar with the Envy products and therefore don't know what model ego system they sell, but the ego style PV is one of the most commonly reproduced and rebranded systems out there. Typically, the Joye eGo is regulated at 3.4v (providing cooler vapor) and has a 650mAh battery. One of the more popular ego systems out there is the kgo system as it is a true 3.7v device (warmer vapor production) and around an 1100 mAh battery. The mAh rating for batteries is important because typically 100 mAh provides enough power for 1 hour of vaping depending on how heavy of a vaper you are, YMMV.
As I'm mainly a cartomizer person I can't really comment on the performance of tank systems, but ego devices have the very common 510 threading allowing you to experiment with different tanks, cartomizers and atomizers without having to purchase all new systems.
The same juices are used in all different types of devices, but the flavor and vapor production may vary a bit. Some of the thicker juices may not wick properly in some cartos while some thinner ones can leak in a tank. Flavor of all the different juices is extremely subjective and can vary greatly from person to person. Finding a juice that satisfies you is the biggest and often times the most exciting part of the vaping experience. Hope that helps a bit.