This is an interesting discussion. It really should come down to your target platform. If you only want mobile then try to spec a format that works well in that resolution. Somewhere floating around this forum someone posted an infographic on native resolutions for all major cell phones. Drat.. I cant seem to find it. Interestingly I started working on my game assests so long ago that 800x600 was my target resolution.... no one uses that anymore. Try to factor in the future too —@ Are you going to be using a grid / tile-set design? If so you might want to ensure the tile size is divisible by the resolution to a whole number. Some topdown shooters use a longer Y axis than X axis if I remember right.
Sorry I'm not much help but maybe there are some good ideas to get you thinking.