On the voice-acting front, both are so good that it's a non-issue. A good voice actor will have you believe the character, without really thinking about the voice. Both accomplish that for me.

I find the female Shepard to be less believable in the role that she plays. In part this might be because of gender issues; some deep-rooted sexism in my brain, maybe. More likely, I think it has something to do with the fact that the dialog and story are written around a male character, and the female character has identical lines. And even though a female character in Shepard's position, doing the things that Shepard does is perfectly fine and believable, I think that this theoretical female Shepard would probably NOT use the exact wording in all of her responses as a male version, or have the exact same facial expressions and the exact same animations at all the exact same times. And I think that that is the biggest source of my not being able to take the female version as seriously in the role: because it's a female voice and body doing things and saying things that were written and choreographed for a male character.

Which is a lose-lose for Bioware, because if they start having the female character say different lines, they'll probably get accused of even worse things. Not to mention the astronomical increase in writing, and thus voice-acting, costs.