It looks like someone has done some detailed testing to show that the translucent mirror in the Alpha cameras does indeed NEGATIVELY impact image quality. The NEX-7 should have better image quality than the A77 since the NEX cameras don't use the translucent mirror (assuming you aren't using that new adapter with the translucent mirror built in).
http://thesybersite.com/sony/a55/index.htm#slt_mirror_image_quality
http://thesybersite.com/sony/a55/index.htm#slt_mirror_image_quality