dutasteride is new, and it's long term effects are relatively unknown. It also blocks a lot more DHT, including type I. The effects of having no type II DHT are well known, because of the Dominican semi-hermaphrodites, who have a type II-5ar defficiency. Also, dutasteride is just scary to a lot of people. Fully 50% of it is metabolized in some unknown way; the Glaxo scientists couldn't figure out where it went in the body. It also has a long half life, and stays with you for a long time. Plus it is not approved to treat hairloss.
For me, dutasteride is the very last resort. I don't like messing with my hormones anymore than I absolutely have to.