Arcane is currently quite flexible with what you can do to pull decent numbers.
Scorch Weaving vs Arcane Barrage
Currently, Scorch Weaving is ahead as you don't drop stacks and 'waste' DPS by building them back up again; in reality even if your Barrage hits like a truck (which it does), it doesn't make up for the time taken to build back to 6 stacks. Therefore, Scorch Weaving is currently superior.
The way Scorch Weaving is done (at the moment, anyway) is by alternating casts of Arcane Blast and Scorch while hovering around a certain Mana % (will come back to this later), replacing Scorch with any AM procs you get in the process. This rotation also varies on your haste levels; the higher your haste the easier it is to make a 'mistake' and Blast once too many times without it being detrimental to your DPS. At high levels of Haste you can Blast twice between each Scorch/Missiles as the regen is decent enough for the whole process to be mana-neutral.
Regardless of what method you're using, it is
always a dps increase to use Barrage if you cannot refresh stacks and would otherwise drop the stacks, for example a period of heavy movement where you can't even find the time to re-cast Blast or Missiles and are just spamming Scorch to maintain DPS - Barraging before stacks drop off is a DPS increase.
Mana %s
Typically, most people are currently going for the full-Mastery build once they have gained the SECOND extra tick of Nether Tempest (3056 Haste non-goblin, I believe). With this build; it's more optimal to hover around 85% mana, slightly higher if possible as your Mastery is a large amount of your DPS and therefore the more mana you're able to keep; the better.
However, some people are currently using an all-out-haste build (like me, for example). There is
little that proves that this is either ahead or behind Mastery build, it's currently just a preference. As we obtain more gear it's likely that Mastery pulls ahead, but for now they are roughly even. In this build you have much more mana regen, and as a result are able to get more Blasts in per Scorches than if you were going Mastery build, and this is where the benefit of this build is. If you decide to go this route, typically you can hover around 80% mana without losing DPS, although I wouldn't recommend going below this % if you can and I personally still try and hover around 85%, depending on what cooldowns are up.
Frost Armor vs Mage Armor
This *usually* depends on preference more than anything. Typically I believe that if you're going full-mastery build you should use Mage Armor, and vice versa for Haste build and Frost Armor, but I know that people are using Frost Armor to hit Haste caps so they are able to reforge more to Mastery and the same the other way; so it really just comes down to personal preference.
I *believe* the haste caps are based on Haste %s, so you can determine whether or not that includes the 5% haste buff or not. You can read more about haste caps
by clicking here.
Hope this helps!