'Line in' and 'Mic in' have a considerable difference in gain... you may need to attentuate the signal before it gets as far as the computer, to avoid serious overload distortion. Insert a 10k resistor in series with each input channel to convert line level to mic. level... simple, but effective.

Another consideration is the inherent background noise a lot of computer sound cards have. This can sometimes be remedied by using the 'DC offset' facility in your software, or by recording 'silence', and using that as a 'template' for noise reduction. How you do this depends upon your software.

I have an old Soundblaster AWE32 (16 bit ISA) which is very 'clean', and I use a Soundcraft Folio Notepad mixer (which has RIAA EQ for vinyl), and Sound Forge. I'm happy with the results, so I'm sticking with it!