I recently had the same dilemma. The CNPS 5700D seems to run quieter than the 6500B but I felt that the 6500B would give better cooling, even the ALCU model, so that's what I went for.
I think the 5700D would perhaps be the better option if you were to direct the ducting fan towards an outtake fan but I don't have one of those.
FWIW, I bought mine from Doc Micro
where the 5700D and 6500B ALCU are both €39 (6500B CU is €53) - all prices incl. VAT. They ship to Belgium. (no affilliation other than as a customer)
I'm happy with the noise level and cooling performance on my P4a (Willamette) 2.0GHz OCd to 2.1GHz with the 92mm fan supplied with the flower at its lowest setting.
Now to research PSUs and GPU coolers. This site is really good for that sort of thing!
Let us know what you decide!