I have not ridden a Pack Rat. I own a LHT, it is my daily commuter, grocery hauler, get-around-town, as well as touring bike. I carry any loads in the rear panniers.
I love the variety and niches that Surly covers... but frankly, I just don't get the Pack Rat. I guess it is Surly's love letter to Randonneur bikes. If you intend to ever use rear panniers on it, then be aware that it has a shorter chain stay (shorter than proper touring bikes) and you need to make sure you can avoid heel strikes to the panniers (although I think its 415mm might be a good length).
This article is written from more of a touring perspective, but is an interesting
read given the large selection of Surly bikes.