Many cats are kept as indoor cats these days, mainly because of owners fears for their safety. They can be perfectly content, so long as the owner takes time to mentally stimulate the cat with games and challenges.
If you are happy for your cat to go outdoors, you are providing a more natural way of life, but please ask yourself how you would cope with the "gifts" of dead or dying birds, mice and young rabbits that would be brought home.
As with dogs, there are expenses to consider. A weekly food bill; Vet bills for routine injections, worming and flea treatment; Insurance or vet bills for illness or accident.
And of course lots of toys and treats.