There is no question that it is better for the cat and for the outside to keep them in. Outside cats kill millions of songbirds every year, and their lifespans are much, much shorter than inside cats. Outside cats are also commonly parasitized (fleas, ticks, worms). Roundworms and roundworm eggs are a common part of an outside cat's poop. When an outside cat comes in, it brings the outside in--ticks, fleas, etc. So unless your cat is a barn cat (which I support because the alternatives for controlling rodents--poisons--are so much worse), it really is better for all concerned that he or she not be an outside cat.
Outside cats go in sand, loose dirt, mulch--anything that sort of texture. And no, you can't really train them to use only one specific area if there are multiple areas of those materials.
We have two cats, one inside and one outside. The outside cat we've trapped inside multiple times, but she was a feral cat for a number of years and becomes extremely aggressive over time. Sadly, as much as I HATE it, we have to let her out during the good weather. We'll be trapping her again once the first frost comes (and at that time I'll have to catch her and worm her and de-flea her and so on). I'll also be able to start feeding the birds again once she's trapped inside. Our other cat is inside-only and has never been outside in his life. He is so much more "our cat." He's more people-oriented, I never have to worry about fleas, I don't have to worm him. I am much happier and more secure with the inside cat than the outside one.