I think the 90mm you're referring to is the Tamron lens and that's known to be a pretty good lens. It'll also give you a lot more working distance from your subject than a 60mm, which is why a lot of people seem to like 90-100mm range macro lenses a lot more than 50-60mm.
The longer focal length (60mm vs 90mm) the farther away you'll be from your subject in order to get the same shot (so like 9inch with the 90mm instead of 5inch with the 60mm). This can be very handy when photographing bugs that can be startled easily. I have a 100mm and I think it is great.