Should I use USB or Line-in for vocal recording?

I am deciding whether to purchase an Audio Technica AT2020 USB condenser microphone or to purchase the XLR version of the AT2020 and run it through a Mixer/PreAmp and connect to my computer using the line in port (with a 3.5mm to RCA cable). I am using a Macbook Pro and Logic Express 9 for my vocal editing and am looking to do some vocal recording for original music in my home studio. Which option would result in the best sound quality? USB or Line-in?