Gem Chests have best value if you are trying to bulk up your amount of cards. You can open up 250 gem chests for the cost of 50 VIP chests. 50 VIP chests will give around 80-96 troops and stones whereas gem chests will yield 250. The gem chests will net a lower rarity average, but you get a lot more for the same cost. If all your ultra rares aren’t mythic yet then gem chests will pretty much always be better than VIP chests. If you have pretty much all ultra rares at mythic and plenty of runic traitstones, then VIP will just about always be better.