What to evolve into
Just got a perfect iv eevee, dunno which one to evolve it into, but I do know which ones not to. Most all of the eevee evolutions dont have match ups against useful legendaries aside from glaceon and leafeon, which is how I generally look at what to evolve/power up, so I left that out of considerations for the others
Flareon; have so many Charizard with blast burn and 2 93+ max attack entei,
Espeon; might be best psychic attacker (nonlegendary), but got a 3 maxed out sb mewtwo and a decent level 96% psychic one from all the trade candies for machamp takedowns
Jolteon; *see magnazone and electavire*
Umbreon: defense isnt much of a meta outside blissey
Vaporeon; maybe if it were 2016
So that just leaves glaceon and leafeon, both of which I dont know which one id need covering more
My grass pokemon are just one FP venasaur (started playing 3 months prior, didnt start raiding regularly until latios/latias so this was before I figured out I might actually want more than one of each CD pokemon) 3 alolan eggtrees and a few solid Roselia and tangla for their gen 4 evolutions with lots of candy for them, but I know water and electric do most of the jobs grass does so I dont know how well their gen 4 evolutions and eventual FP sceptile (and leafeon itself) stack up compared to things like gyarados/kyogre/(eventual) HC Feraligatr against things other than water types that they share effectiveness against and how those grass types also do in comparison to electivire/magnazone against water types
For my ice types, presently I have none aside from 8 eventual mamoswine (6 Swinub and 2 piloswine, nub's level 20, one pilo level 25) with only around 1.1k candy, 1 or 2 sneasel for weavile
I have a feeling both are outclassed (I know for a fact mamoswine outclasses glaceon) but I guess the question more so it which of leafeon and glaceon is less outclassed? if that makes sense, but that seems to be standard for eevee's evolutions these days
Answers
Mamoswine outclasses Glaceon pretty heavily.
Among grass types Leafeon is outclassed by FP Venusaur, Roserade, and in an assumed 4 months FP Sceptile. Also Shaymin-S but lolmythical so who knows when we'll get it. Not to mention grass type as a whole is outclassed by either water or electric types except in cases of a double grass weakness.
Between those two it comes down to personal preference given what you already have/will have. Another option is t reconsider Espeon since strictly speaking it does have higher DPS against fighting types than C/SB Mewtwo, albeit more squishy.
Or just go with your favorite Eeveelution and call it a day. You sound like you're content with what you have so you could end up sweating out which one's the best bang for your buck only to never use it.
Im not talkign about do i want to power up this electric type because of vikavolt or something, this wondering what to do about something that can happen literally any day.
If I was doing any gen 7 preparation cult or whatever, I wouldn't even be on here. theres optomizing for something that can come out in what, less than a month and something that can come out in 3 years at best
If youre going to say something at least make it accurate
Y'all don't get it. Eevee is the catchall for beginners, a stepping stone. Evolve one for a mere 25 candy, and you'll get a half decent attacker in one of three types, with another useful one on tap if you just walk it 10 km. Helps you get from your first Pidgey to your 6th Mewtwo. No one expects a level 40 player to be using hordes of eevelutions - they should have all of them beat with better options (except maybe Espeon.)
Further, aside from the name trick (which I assume we'll have available to us for Glaceon and Leafeon), it might be the case that you evolve with a Sinnoh stone and you get one of the two, so you may not have this choice if you go Glaceon/Leafeon. Or it could depend on the weather when you evolve. Or a lot of other things.
Level 40. I have 2 Espeon, and then one each of the first three max. I never use Vaporeon, but I do use Jolteon (Discharge!) as I'm light on Raikou and Zapdos and Flareon, as I didn't have a great Charizard Day, nor pick up decent Moltres. The Flareon and Espeon make good 4th+ mon in a line-up of 6, and Jolteon normally leads off.
I asked the same after the Eevee community day and was recommended to save my 100% for Glaceon. Back then the moves of gen4 mons were not known but even today i feel it's the closest competitor for top of their type among eeveelutions. The strenght of eeveelutions is that they are good and readily avaible but not top tier (and power creep will make that even worse). Or you could rather go with your favourite eeveelution and call it a day instead of driving you insane :)