But i've been told the MKII isn't accurate enough, that actually the BE translates bass better or something like that.
Here is what I would say, not from experience, but from what I have read about people who mix have said on here.
Over in the beats hate thread some poster was saying he uses them to mix with and that they work better than the AKG 701s he had, hell they could've been the same Annies that I have. In general everyone would tell you that the beats suck as reference and the bass is bloated and that the 701s (more so even Annies) are great reference headphones. The thing is, the AKGs lack bass impact even though they pick up the music notes quite well the impact just isn't there. When one poster posted some tracks he mixed with both it was noticed that the AKG tracks were quite a bit more bass heavy. The reason for that is clearly that he was looking for impact in his mix and he had to way over boost the bass to do so. The mix was much better with the Beats because they are more bloated and give that impact he was looking for so the mix is much milder in the lows.
Now how this relates to you: If you tend to add too much bass in songs and want to feel impact in there then I would go with the BE because it will be a more mix and will let the hardware decide the bass response and will keep everything nice and tight. If you like more bass in your songs and are concerned with not putting enough in then the MKIIs would be the better bet since it will most likely cause you to naturally add more bass to the mix. This is superior if you want to optimize for the general listener that wont have good gear [not optimized for those with beats (or other bass-head phones) though, which seems to be everyone...] However, it may give them even more bass boost which they will probably love.
I will say this: if you mix a song on the MKIIs and listen to it on the BEs, you will likely find that bass overpowers a little bit or more than you expected. If you mix with a BE and then listen to it on a MKII then the mids will sound a lot more prominent and the bass will be a bit subdued in comparison. This is because mixing with one of these phones and listening to on the other will basically double the effect of any differences between them.
I went with the BEs and I love the Annies so it was a bit of a contrast whereas the MKII more of the same but I dont mix music either. My thinking was mainly that if the MKIIs were spot on then there would have been no need to make the BE. The only complaints the MKII every really received was the bass and bam you have the BE. The only real downside is the slight recession of the mids. In general for mixing and I believe the soundstage to be the same, based mostly on fit, but the MKII could have the ever so slight advantage with the extra bass slightly damping the stage a bit. Instrument separation and stereo effects are still more or less on the level of the AKGs.
Given my brash and rash assessment of your mixing style completely based on that one sentence statement I think the BEs will provide a better result on higher end systems and the MKIIs better on lower end. The BEs will give a more pure mix and you will end up slightly bloating the bass if you use the MKII (I use this as a negative, you may be into it).
You cant really go wrong with either, great price great performance.
Hell pick up two, or even better both and come back with rave reviews and in depth analysis, we'd love you for it you'd be a mini hero and we would all have to listen your music lol.
I hope this helps!!