I pick male characters when
- i can define their background and get lot of influence on their actions during the course of the story(if there is one, heh...)
- i can identify with them to certain extent: random 'murrican military dude in space - "meh", a cross between Tomb Raider(adventurer type) and Tony Stark(inventor/science guy) - "nice!"
Most isometric RPGs with elven mages in them i try to play as a male elven mage.
I play females when
- the game is mostly about gameplay mechanics and pretty graphics, less about story, ie most MMOs, Diablo3, shooters
- i don't find the main protagonist interesting as a person
- the character editor allows me to create a female protagonist looking like a cross between a porn star and an olympic athlete with a really pretty face, ie some korean MMOs and Skyrim - i simply can't resist
I played a female character through ME series and i'd like to try a male one this time, but it will depend on what a person the protagonist will be and how the customization will turn out.
Also, voice acting is indeed pretty important. I noticed this when i tried to play a female sorceress in Diablo3 and had to create a male wizard instead because i couldn't stand the tone and intonation of the female voice after a while.