There's no difference between losing 5 lbs on your body vs your bike,l except that losing 5 lbs of body weight will make you feel better in everything you do while a 5 lb lighter bike will only feel better when you're riding up hill. And the "rotatating weight" effect is so small that it is meaningless.

When you're as lean as you can get then worry about getting a lighter bike.

+1 for riding more.