I guess it all depends on where you want the less pain. If you hate ass pain then use shoulders, quads, tris...
If you hate the pain associated with glutes then inject somewhere else- just beware that you shouldn't inject more cc's into a particular sight than recommended. Also you should consider the possibility of sight enhancement with regard to injects. In other words the possibility of the particular sight you are using for injects growing more than your other bodyparts. I don't completely subscribe to this possibility, however I believe it may have a MINIMAL effect.
Personally I used shoulders and quads for my first 3 cycles but now use only ass and quads. For the amount of gear you are gonna be shooting I would use quads and shoulders if worried about ass pain. At the same time, if you are using glutes and it is painful there you might have more pain in shoulders.
In the end it would be prudent to experiment with each inject sight to see the level of pain, any leakage from the inject, and effectiveness to see which might be most efficient for you.
BTW I'm not worried about what others say- I would definitely inject both sust and deca twice a week. The sust is more easily justified for this dosage scheme but I'm extremely anal so I would include deca, EQ, primo, etc. twice a week as well. Best of luck bro.
SwitchedOn