Is it more ecological to use the in-sink garbage disposal or to use the trash for table scraps?

Trash. If you feed the garbage disposal, the sewers clog up and eventually have to be dug up and replace at great inconvenience to you, your neighbors and with a great deal of energy.

Of course, food scraps will take a very long time to decompose in an anaerobic landfill and would produce methane in doing so but if you can't compost it yourself in a garden compost pile or send it to a commercial compost pile which can handle a lot more items than a garden compost pile, then the landfill is the next best destination for food scraps.

Other than composting, I would say throw in the trash. Garbage disposals use electricity and require water. When thrown in the trash, the scraps will go to a landfill where it will decompose and become a fertilizer if you will.