im f2p so i have it only on paper, cant test myself but i know vip chests have better chance for mythic then the gem ones, even after recalculating the costs.
i already reached a point where i have almost all the legendary cards (missing imps) and all lower rarities missing just traitstones and natural mythics. at this point i perceive mythics as most valuable and highest goal so i would go directly for the chest that has more chance at it which is vip.
but since i dont have vip lv5 i cannot really tell would i not change my mind if i tried it.
about the actual drop chance you can calculate from this: