Winner of Coffee Geek’s 2006 Editor Choice Award for the Best Training Tool category, the Espro Calibrated Tamper features world class construction. This unique tamper automatically clicks when 30 pounds of force has been exerted onto it, alerting you when you have reached the proper amount of tamp pressure for espresso extraction. These Espro tampers feature stainless steel convex bases and are available in 53, 57 and 58mm sizes. The 53 and 58mm tampers have polished black handles. The 57mm tamper comes with a red handle. Espro Calibrated Tampers are designed, machined and assembled in Vancouver Canada.
Why would I choose the convex vs. the flat base?
A great question with not a right answer. It has been a debate for a long time and both sides have valid points. Read this -http://www.coffeegeek.com/forums/worldregional/australasia/148871?Page=1
I think it is you buy whatever philosphy you would like to believe. I have always been a flat guy, and probably always will be.