Well, if i had to name a reason why not take both: sea of flames isn't exactly a stellar ability even at it's best, and if you have magelight running, then the SoF you just slotted is even more gimped because it's 10% spell crit does not work(it does not stack with magelight).
I would only slot both it if there was nothing better to put in the second slot than a hands-off single-target DOT that is a bit on the weak side.
More mana means bigger pool to dps with... dps is also about sustainability.. also inferno and it's morphs is free dps... sure it's not omg wow good but it's a free single target spell every 5 seconds... it adds up
I've experimented with both extensively. The only instance I found Sea of Flames worthy of a bar slot was an outdated build I used with pure DOT damage used solely to achieve the maximum Valkyn Skoria procs. With the nerf to the set, Inner Light has become the new DPS standard for DK's for a Major Prophecy buff. With 35k plus magicka being an easily achievable number in today's meta, the 5% magicka buff outweighs the DPS provided by the minute DOT of Sea of Flames.
Just going to say that max magika effects how powerful your spells hit. At v16 1045 max magika is equivalent to 100 spell damage. Use inner light to see how much damage a particular spell goes up and determine if you'll get more damage using that ability or more damage out of a fireball every six seconds
For pure single target, you run both at same time. Might be counter intuitive, but it works. Outside of trials, I almost never run both, because I like the extra 2 skills I can slot. I believe the SOF is slighly more damage than Innerlight, but its pretty close. It comes down to a passive DoT vs more magic for extra damage on everything else. I have gone back and forth. I personally like inner light better for 2 reasons. First, I just like the look of it better. Second, I find my SoF tends to aggro things on accident. Neither of those reasons are particularly persuasive I grant you, but that's what I do.
Edited by Oreyn_Bearclaw on January 13, 2016 6:06PM
I use Inner Light most of the time. 7% max magicka (5% from skill, 2% from MG slot passive) and MG regen passive is quite useful, and for trash mobs, it works out to be the best (Inferno affects just one target, while IL boosts damage to all targets, and most targets don't last long enough to benefit from Inferno's DoT).
But in specific situations, I use both. E.g., single-target boss fights and trials. Yes, I am losing a slot on both bars, and yes, the spell crit is redundant. But that doesn't matter if I don't have something more useful I could place on my bars. I have an addon that lets me swap preset bars quickly, so if I'm at a tank-and-spank dungeon boss, I'll swap to my setup with both IL and Inferno, and swap back after so that I have more utility spells for the next trash pull.