So after playing for about 6 months now what I first believe to be fact I am starting to question. Up until now IG's were the choice over bulls without question. The pro I believed to be important were: ST6 wounds easy and rips though armor, ASL isn't that big of a deal when your IN is 2 anyway, and LD 8. Now after many play sessions, I have to say what I thought was important or unimportant is very different.
Starting with ST6. Its nice certainly, make wounding T4 on 2's and -3 to armor. However, ST6 opposed to ST5 seems like a win more condition. Being that the game system is set up to where you always would no matter the toughness on a 6 added with the fact that armor is hardly important with the amount of ward saves that exist and if they do have armor a -2 either removes the save or greatly hampers it. As a side note I can't tell you how much it pisses me off to be ASL with ST6 and the unit I'm fighting have IN 1 or 2 with light armor or none at all.
ASL when IN is 2. IN 2 is still quite low. Most everything is going ahead of you granted but its more than annoying to be ASL against a unit that is IN 2 especially if they are IN 1. Plus this is kind of an point that is connected to the ST 6. One exists because of the other. Being that my feelings are that ST6 is just win more compared to ST5 and it costs ASL to get there, well ya not so worth it.
Leadership 8. The difference of rolling leadership at 8 compared to 7 is certainly a BIG difference. Though as time has gone on I've realized you are forced to run in a giant horde-leadership-BSB-bubble anyway. So this becomes irrelevant because of that.
As you can see what I thought the pro's of the IG were seem to be pointless or overkill.
Now onto the pros of the bull.
Same armor as the IG with the added parry save. Its only on a 6 from combat in the front, but it makes a difference.
You probably have noticed how I have kept refrencing ST5 above and not ST4. That is due to the fact that I'm taking into account of having a lvl 4 Maw SM. With a lvl 4 you are all but promised the +1 St spell. A ST5 bull is a powerful force with added durability of the parry save and being 9 points cheaper. Its hard for me not to want to take the seemingly more efficient unit for a cheaper cost.
Obviously I will need to play with them for a few months to give my final stance, but I know much more now than I did 6 months ago in regards to mechanics of the game, what matters, and the other armies. From the knowledge I've accumulated, given the correct support I feel the bull is a better choice in most situations.