It's better to scale in the 2D drawing. In 3D you will get bad quality.
Sometimes it could be better to scale up in 3D but you shouldn't scale down.
Scaling is typically faster in 2D since there is less geometry to manipulate. If you think of your 3D model as your base line, you'll want to keep them at original size in the event of having to combine images from multiple models. Once you star scaling your 3D models, it's real easy to lose track of the various scales you had used in the past. Hope this helps.