I've got an '03 STS... That would be my choice... But find a 2003 and check for the (Z Rated Tires) version which should be indicated inside the drivers door panel.
By 2003 they nailed all of the NorthStar type things... Have a buddy who is a car dealer, and I paid dealer wholesale... Got mine in 2005 coming off a 2yr lease, with 25K on it. It now has 66K and the only issue I've had is front struts.
Struts are expensive, I fortunately have an extended warranty on it, but haven't had to use that for anything else. If I had to support the cost on my own, I would have converted them to the Monroe replacements. Still $500 each.