Decals are nice if you just want to be sure to maintain the proper aspect ratio and size of the decal, or for their speed to just plop them in real quick and work just fine. Overlays give you more control as you can resize and rotate them; decals cannot be rotated or resized.
Overlays are slightly more expensive though. So rule of thumb is: use decals if it doesn't have to be resized nor rotated, use overlays for all the rest.
I think the difference in expensiveness is quite small anyway.
If this is your first time logging in after the migration (Feb 8, 2022), you must reset your password to log in. Follow this guide if you're having trouble