Not only is it difficult to get a modification (because of the sheer inability of mortgage companies to cope with how many are needed – see my last post), but getting a modification may result in your credit score dropping anyway. See today’s article in the Statesman (link).
I need more information before I would start recommending against modifications. I still think you should try hard for a modification before turning to Chapter 13 most of the time (see my post To 13 or Not to 13). Even taking this Statesman article into account, the credit hit is not likely to be as long-lasting from the modification as it is from a 13.