+1 HH
What the OP is looking for is not just IBU's, but the balance in a beer. This balance is the relationship between IBU's and initial gravity, or the BU:GU (bitterness units:gravity units) ratio. This is a ratio between the IBU's and the decimal portion of the OG. For example, a Pale Ale with IBU of 40 and a OG of 1.060 will have a BU:GU of 0.66.
There are other thing to consider also, most importantly what hops are used and how are they used. Some bittering hops are considered harsh, namely Chinooks (Mmmm Chinooks...) and can really assault the tongue. Others, like Magnum or Warrior, are clean bittering and tend to balance sweetness with out the harshness.
As HH mentioned, dry hopping is a huge part of the aroma and flavor profile of an IPA. Sierra Nevada tends toward Cascades, very citrusy. Harpoon, IIRC, uses Ahtanum, which has more floral notes.