Echoing Strike & Fire Warlock Questions

I have a bad Arioc's (+2 skills) and I just realized I have the runes I need for Obsession besides Lem. I know it isn't highest dmg potential, but wouldn't it still be awesome for this ES build ? I still need a Vex for BotD, should I do that instead? My goal is fast kills plus steady survivability.

I'm also wondering if a high end Fire build can get through end game content effectively. I think I like the fire gameplay a little better, but I haven't played it to nearly as high of a level so I dont want to Respec and Gimp myself if It is significantly worse than ES.