Avoid bowden extruders if you're printing with flexible material. Retraction and extrusion can become inaccurate due to stretching and compression along the long axis of the filament. 3mm is also significantly easier to print with than 1.75mm as the filament has less tendency to flex (if that makes sense). I've had problems printing with 1.75mm PLA where the filament squeezed out around the hobbed bolt, rather than extruding properly. Changing to 3mm cured it.