I've geared my SV up 1 tooth on the countershaft to drop cruise RPMs on the hwy. Worked well for the SV, as it has plenty of torque to pull taller gearing no problem. Generally you can expect about a 500 RPM drop from 1 tooth additional on the countershaft. Same thing can be accomplished by reducing the number of teeth on the rear sprocket (usually about 3T on the rear), but generally it's less expensive to do the countershaft and the result is the same (as long as the ratio is the same of course).
You have to balance your gearing for your factors. Do you do a lot of hwy riding and want a lower cruise RPM - does the engine have enough power on the hwy if you do? Are you a large person and need more torque? What are your engine's characteristics - will you be dropping the RPMs into a flat spot in the power curve if you alter the gearing? There's no one right answer for everyone. You need to consider all the factors that you personally are working with.
As far as sprockets go, I was impressed with the Sunstar rear sprocket my son just bought for his Ninja. It's black, made in Japan, and was reasonably priced.