Originally Posted by smarkinson

The angle of the top tube has nothing to do with a bikes geometry. It's perfectly possible to build two bikes with the same geometry but one with a horizontal top tube and the other with a sloping top tube.

You should be looking at things such as reach and stack and comparing these against your existing bike. The stack is the height between the BB and top of the head tube. This will affect how low (or high) you can set the handlebars. Endurance type bikes tend to have a longer head tube than a more racing style bike, maybe 1 or 2 cm.