I’ll try to answer with my limited knowledge, if a theorycrafter or a mechanic expert sees something wrong in what I’m about to say, feel free to correct me.
Crit Damage :
as a rule of thumb, crit damage is more valuable than X% “Any damage type” Damage. That is because you’ll get a lot of X% “Any damage type” Damage from your gear, so +50% Damage is not that much of an upgrade when you already +2000% Damage, which is kinda the norm in the endgame.
Of course, it also highly depend on your Offensive Ability number. 2800 OA is roughly the minimum goal for engame build, but for crit to really shine, you should aim more toward 3100, and if you build does rely on crits, 3300 is much better. But it all depends on the build.
So in essence, you can’t really compare stats in a vacuum, that’s what makes creating build so complex and freanking interesting.
But just taking your example, if I had a chance to choose between +50% Elemental Damage and +50% crit damage, and I was able to reach say 3000 OA, then +50% crit all the way! Hell, I’d probably choose +10% crit damage over +50% Elemental.
Magi vs Amatok :
Same as above, you can compare two devotion in a vacuum, but it’s of limited use, because you also have to consider the devotion path in its entirety. Sometimes, it’s worth taking a constellation that doesn’t stricly fit you build because it allows you to take two Tier 3 constellations instead of one, or you spend less devotion points for unlocking and have more free points to take juicy nodes.
Magi requires 10 green, has 7 nodes, and gives you 3 green. Amatok requires 6 blue and 4 green, has 7 nodes, and gives 1 blue and 1 green. This means that Magi is more efficient, and in an Elemental build, you’ll generally aim for Blind Sage or Attak Seru, both requiring lots of green. Also, Blizzard is an “on crit” proc whereas Fissure is a regular “on attack” proc. It should mean that’s it’s easier to make Magi work than Amatok, since you need high OA to really benefit from Amatok.
But in general, neither should be your endgoal, they’re nice to have, but do not strike as essential constellations. Most important constellations are probably the ones that give a -Resistance Reduction% proc, and sadly, there is none for Elemental damage, but Rhowan’s Crown is good and the proc has a very nice flat -32 Elemental Resistance Reduction.
Roguelike vs Shattered Realms :
Shattered Realms is better for untargeted loot, you go as high as your build allows you, then you make two shards runs for max efficiency (75-76, 65-66, 50-51, whatever you can). Due to the abundance of mutators, and boss rooms with several bosses stacked, it is a bit of a stressful mode with varying difficulty that will keep you on your toes.
Roguelike vary much less because they have less mutators, so they’re more predicable, and in general more relaxing (depending on your build performance of course). There are also items than only drop there, the most notable ones being Epic and Ledengaries that only drop from the end bosses (Monster Infrequents or “MI” are green items that only drop from specific ennemy types, well, these Epic and Legendaries are MI that drop from specific bosses only)
For exemple, Alkamos, the last boss of the Steps of Tourment (considered to be the easiest Rogue-like dungeon by most players) has two epic rings, two legendary rings and one legendary scythe that only him can drop.