If you don’t have VIP chest access, in regards to mythic collection alone and nothing else, Gem Bounties are always slightly more efficient. For the Monday-Saturday packs one mythic “costs” approximately 8333 gems worth of Gem Bounties, such that if you keep buying Gem Bounties, you will eventually end up with one “extra” mythic for the cost of 8333 gems. You are estimated to get on average get one mythic spending 9k-10k gems on gem keys (1/1000 estimated drop rate), but you’ll get much less fodder drops (a ton random legendaries to ascend and traitstones from vs a very small amount of jewels from the bounties that convert to a handful of celestial or half that many arcanes). Therefore, I consider it a matter of personal preference if you don’t have VIP access, since there are good arguments in both directions - avoiding RNG, slightly less cost, and having access to old mythics by going for Gem Bounties, getting a ton more fodder by going for gem keys.
If you do have access to VIP chests, VIP chests will get you more mythics on average for the same expenditure by a pretty good margin - an estimate of on average 1 mythic per 4500-5k gems spent (100 VIP chests), and spent during an exclusive period, still always a new one - as well as far more fodder drops (legendaries, arcanes, celestials, gold). Still a matter of personal preference, but the lines are much clearer - if you really want old mythics, and you want to get them quicker, or just want to circumvent RNG and don’t care that you are giving up almost half your potential mythics for the same “cost”, or you have enough gems that they don’t approach scarcity and can do both (bracket 1, top 3 guild wars), you should still go Gem Bounties. I personally don’t see these as a good idea for VIP5+ players over VIP chests otherwise, and especially not long term.
The Sunday pack is completely different, since you get 80 diamonds and 400 shards and two gem keys for 50 gems. You should always buy this one if you are at all interested in crafting mythics, regardless of VIP level.
@Grundulum - I leave these windows open in the background and come back later. Your mention notification reminded me I was working on a post