It depends why you need to slow down! If you're coming up to a junction where you may need to stop then engine braking and changing down in good time will probably do the job quite adequately as well as giving you time to assess what is going on ahead. Get it right, and all other things are equal, you may not even need to stop at all - unless it's a STOP rather than a GIVE WAY junction. Roundabouts are a favourite for this, especially on a motorbike.
There are times, however, when using the brakes is to be preferred even if engine-braking would otherwise suffice; when there's someone behind you and you need to let them know you're slowing down by showing a brake light.
On the other hand which is cheaper to replace, brake pads or a clutch disk? You pays your money and takes your choice.