I bought an 08 Spec B 6 speed when my 5 speed snout got chewed off by the TOB. At that point I was looking at either buying a replacement 5 speed or upgrade to a 6 speed. I have never ever had any regrets about going with a 6 speed. The Spec B gear ratios are awesome. In 6th gear, I am at 2600 rpm at 70 mph. I immediately saw a 2mpg increase on the the highway. 1st - 5th seem very evenly spaced. the STI 6 speed is nice as well, although you lose the taller 6th gear, depending on the year.
I realize that the Spec B tranny is not on par with the STI 6 speeds because of the differentials, but it is very quiet and smooth. I chose to replace my rear diff with a WRX 3.54 R160 so that I could retain my rear axles which were fine and still are. I purchased the transmission from an auto dismantler that had just recieved a wrecked spec B. I bought the Transmission, shifter linkage, drive-shaft, slave cylinder, and clutch in one package. I believe the bill was around $4000 for everything including installation.