For me, this was a tough decision, I felt arcane was underpreforming for me, maybe it was I wasn't playing it right, even though I knew how it isn't just a 2 button spec, I defended how arcane actually can be more complicated then just smashing AB. Now it wasn't that I was doing 5k dps, it's just that everyone in my guild out-geared me, and they still do slightly. It's just i'd end up at like #11 by the end of a fight, and I don't like that. Regardless if people out-gear me or not, there is no patchwerk style fight in firelands, there is movement, somewhere there is movement. No matter what you will end up moving, except maybe beth'tilac where my dmg/dps was #3. Most fights have some movement.
So I looked at the BiS dps.
Fire (33,655 DPS)
Arcane (39,480 DPS)
Frost (37,743 DPS)
Now you see frost behind arcane by a little less than 2k, yes, we know, we've seen. But how many people are going to have SoW to start, or get the legendary. Right now I have neither and am on a course to have neither since I am new to my guild. I took my gear, and adjusted it frost vs. arcane, in rawr formats, with changing one peice aswell, my chest from t11 to t12, as of now I am going to get my 4 set next week, I am choosing the chest because I could use the hit so I have plenty of room for new gear without hit on it. I compared the dps gain on rawr between the peices, now it's not 100% accurate, but I went with it and still chose the chest thats t11 heroic over tier 11 regular legs, I know, but it's for the hit.
So I ran simcrafts and in a patchwerk style fight, in my gear as best as it could be as arcane, and then as my gear as best as it could be frost, I compared results with and without the tier 12 4 set, this was to decide if I was going to go frost right away or wait (arcane will still be my offspec) And the dps in a patchwerk style encounter, frost was slightly ahead. Like I said there is a clear lack of these patchwerk style encounters. So I did helterskelter, the results were much different. Helterskelter obviously has a lot of movement, some fights you may not even move as much as it simulates. Arcane lost 11k dps (it was 27k for patchwerk)for me, on a helterskelter compared to a patchwerk. Frost only lost about 6k. (From about 27k) Oh and the difference between having tier 12 4set and not was minimal, since I lose tier 11 2 set, which is 5% icelance crit for frost, and crap for arcane, and the t12 4set is good for frost but also kinda crappy for arcane. So you lose good and gain good for frost, but lose crap and gain crap for arcane. So the difference was minimal.
This is my conclusion, once you start gearing up with tier 12, and get a good chunk of gear thats i378, frost starts to catch up a lot more to arcane especially with tier 11 4 set broken. I'm sure there are others out there that want to go frost but think arcane is the only way to do high dps, it's not. It's your gear. And unless you have SoW or the legendary staff, frost may be better for you for movement based fights. And thats what I found, I have to test it in raid yet, but I think it will definitely be useful on a few fights where arcane does not always pull ahead. (Shannox im forced arcane for the raid, I'd prob be frost every other fight except maybe beth'tilac, aly I am not flying, and frost mages have ice barrier, say hello to a lot less dmg taken, get reactive barrier and drop 2 useless points, it could really make a difference in you becoming the person that takes the least dmg, helping your healers mana too.) My last finding also was, 10m, arcane does better, frost relies heavily on raid buffs, so in 25m frost may excell and 10m arcane has always blown everyone out of the water for me.
TL;DR If you lack SoW, the legendary, you have some firelands gear, and t12 4set or close to it, frost may be a best bet. Using simcraft can always help, it did me, so now I have dropped arcane as my mainspec to frost, I'm just hoping it does well.
Also hoping people who are also confused about frost or arcane look at some points i've hit here so you see arcane isn't always 100% best.