1mg of Finasteride reduced serum levels by 70.8% in a blind study, while Dutasteride at 0.5mg reduced serum levels by 94.7%.
Excerpt:
The results of this study support the hypothesis that suppression of both 5α-reductase isoenzymes with dutasteride can result in greater and more consistent suppression of serum DHT than that observed with the selective inhibitor of type 2 5α-reductase, finasteride. The higher doses of dutasteride (2.5 and 5.0 mg) induced almost complete suppression of DHT at 98.4%, compared with 70.8% with finasteride, consistent with other studies reported in the literature for finasteride (21, 24, 25). The lowest maximally effective dose of dutasteride (0.5 mg) resulted in DHT suppression of 94.7%.
Source:
https://academic.oup.com/jcem/article/89/5/2179/2844345
All articles suggest around the same suppression (+/- 1%).
In my personal experience, Finasteride was much more tolerable, with side effects almost non-existent. Dutasteride (at least from my 3 week experience so far) has come with a slight decrease in erections and libido.
Most studies suggest that Finasteride has a significantly rare occurrence of decreased libido (with consistent use), whereas Dutasteride has a much higher frequency, though it also seems to return after continual use.
Excerpt:
A number of studies have looked at the problem of side effects caused by finasteride.[6–11] These studies which are discussed below reveal that sexual adverse effects occur at the rates of 2.1% to 3.8%, erectile dysfunction (ED) being the commonest followed by ejaculatory dysfunction and loss of libido. These effects occurred early in the therapy and returned to normal on stopping or over a time on continuous use of the drug. The only causal relation between finasteride and sexual adverse effects is decreased ejaculatory volume because of predominant action of DHT on prostate.[5]
Source:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3481923/
So, as you can see, it's an incredibly rare occurrence with Finasteride (regardless of what people will suggest), far less than that of any AAS we consume. The key with these drugs is consistency.