As said above its really up to what drops during your raids which will dictate your purchase each week. I personally think its a huge waste to buy the chest piece when it drops in turn 1. Same could be said for the shield, however the shield is only 300 myth, which makes it the cheapest option. Besides you may not get the allagan shield for a while. Beaten T1 7 times or so so far, havent seen it once, but I did get my chest, yet Ive only seen the chest that one time. I really cant see spending 825 on a chest piece when a chest drops on T1, even though the myth chest is BIS. But I digress.

What I do is check what I currently have, then see what doesnt drop until Turn 4 and fill that out first. So at this point I am filling the turn 2 stuff because I already filled my turn 4 drops with the myth gear. I am not really worried about BIS so that makes my decision making process easier. I figure if I have all level 90 gear, im gonna be a beast regardless.