Many of us have asked the same question.
I started a similar thread, where I actually try to do the math and figure out which is better where. I even have a spreadsheet that will let you see who will do more or less damage and take more or less damage. It's not in a fully finished state, but it will let me draw some conclusions. Feel free to download it and play with it
http://www.ogrestronghold.com/forum/index.php?topic=22399.0After building this up and the lively discussion I have had in that thread I have come to this conclusion: It matters what you need that unit to do.
If you need an anvil unit take bulls with ironfists.
If you need a hammer unit take ironguts.The lower points cost, possibly striking first, and parry save from the ironfist make the bulls able to take the hits and keep standing through the rounds of combat. This ties up that hard hitting unit across from you so you can get a flank charge in or just be able to not worry about that unit for a while.
The higher strength is SUPER CHEAP points wise for the ironguts. Along with (as already stated) the ability to bring a magic banner, these guys can decimate many of the tougher opponents when they hit them. GW has done a great job of balancing that bonus with less than stellar defense against hard hitting units and making them always strike last with the great weapons. This means most times you will get wounds taken off before you even get to hit them possibly removing attacks from your unit before you can attack them.
Ideally, I think 2 units of bulls for every one unit of Ironguts is the best ratio. But the bull units get filled out first.
A lot of people see it this way. Bulls are only 25% cheaper in points than the same number of ironguts. 2 units of bulls is not worth 1 unit of ironguts points wise if they have the same number of models. However, 2 units of bulls and 1 unit of ironguts (same number of models in each) is worth the same amount of points as 2 units of Ironguts. What really matters is the points you are willing to spend and what actions you need the units to perform.
My personal preference now is to first decide how much gnoblar chaff I want for my core, then split the rest of the core points down the middle and bring as many of both bulls and guts as I can
in even points per unit. This strategy will always give me 4 bulls for every 3 ironguts. Doing this lets me bring more of the specialized units like leadbelchers and mournfang, and ironblasters, and well, the list goes on and on.
I usually try to aim for either 9 or 12 in a unit and If I am working at higher total points I will just bring more units at those numbers instead of large hordes. With at least 1 unit of each and plenty of chaff I can see where he puts his hard hitting units and deploy the bulls to counter them. Then I can deploy the ironguts where they can do the most damage against a unit that will shred like paper when my guts even glare at them.
In conclusion: If you are facing a unit that is high strength you want bulls to absorb that hit, if you are facing a unit that is high toughness and/or saves you want ironguts so you can get the attacks through.