I would like to know what is the better choice. I guessed that Archon beam get dmg directly from your DPS, isn´t it? So the higher shown DPS in your "inventory" should be better. And slightly higher is with Emeralds. But i saw lot of people saying that they do more dmg with Ruby...
I would recomend you read this thread, and this and this and check your gear/dps gain on an online simulator as D3UP.com.
That said, in my humble opinion, it is a matter of perception/timed runs. Sometimes the ruby will net you more DPS (and Archon benefits directly from this, as it spread its damage along 60 ticks / second, if I'm not wrong). In this case, you should go with a ruby, no questions asked.
Another times, the sheet DPS will be greater with an Emerald. This is where the "perception"/"timed runs" kicks in. See, when you use a Ruby, your MINIMUM damage raises a lot, and often it's only necessary a short beam to kill mobs, since all the ticks of the Archon Ray of Death ™ will guarantee a minimum damage higher than with the Emerald.
Damn, I could use a graphical representation of this here.
I switched to Ruby a few days ago. My general perception is, while my DPS dropped 6k, my MP8 runs are taking less time to complete in general. Storm armor procs are almost always "1-shot-1-kill" on mobs, and mobs melt away faster.
But this is my case. You could feel it different on your char.
Edit: (As a comparison, my usual Act 1 runs were taking 20min to complete with Emerald. And with the Ruby I'm taking 16~17 minutes).
Bribous: you're right that a ruby removes damage spikes and increases min damage. But I don't get your example... on MP8? And 20% faster? I really doubt that. It must be something else coming into play here. It's not like 250k DPS one-shots anything on MP8.
TheBonhart: all the theorycrafters (see thread I linked) kind of concluded that for the most part it's best to go with whatever gives you highest sheet DPS. But it might vary from character to character, as Bribous' examples shows.
In extended combat with sustained damage over longer periods of time, you want higher sheet DPS.
Which one has higher sheet DPS for you? You just said emerald.
Unless you kite in circles for every extended elite fight, you want the one that provides higher sheet DPS.
Edit: if you take ~5 seconds to kill packs on MP7, you'll take about 15 seconds to kill packs on MP10, if you don't kite. I qualify that as extended combat. Even more so if you have to shift positions or go against Extra Health.