Line 6 TonePort GX question

I am thinking of getting the Line 6 TonePort GX to record 2 guitars and a bass.
I use Logic Express 7 and a Macbook Pro Core 2 duo.
I currently plug the guitar or bass into a Line 6 tube amp, and go into a Behringer 4 channel analog mixer, and then into my Line-in.
My question is, will the following sound better:
Guitar or bass into the Line 6 tube amp, and then into the Line 6 TonePort GX and then into my computer's USB.

Sound quality wise - Can i use this toneport to replace something like an Mbox to go to my computer in digital?

Am i better off going into the analog mixer and not bothering with the Line 6 unit?

Thanks!!!
Stew