Shadow Priest or Destro Warlock?
My Priest isn't doing so hot in LFRs. I'm normally sitting at 8 or 9 on meter and I've been as low as 13 on some of the movement heavy fights. Some of that may be that I'm not playing him optimally because I struggle a lot with multi-dotting and keeping focused on the main target. He's also not as well geared as my lock (ilvl 488 vs 500).
Overall, gear and skill disparity-between both of my toons and other players'- makes this anecdotal evidence mostly irrelevant. I'm positive an equally geared, welled played Spriest could kick my butt in DPS against my Warlock just like I can beat a scrub playing a DK poorly.
As a direct answer to your question, my understanding is that Destro is well ahead of all Warlock specs and Spriest, until you get your tier 15 gear, at which point Destro drops a lot and is barely edged out by Spriest. Which, of course, means very little in reality because none of the current raid bosses are patchwerk style and nobody's perfect.
In short, as usual, the answer to these types of questions is: play what you like most because things can change at any moment.
If shadow sucks....well then hope you enjoy healing.
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