Please help...Which Looping Pedal?

["Carl McClatchey" <cmcclatchey@amlaw.com>]

I am looking at the Akai Headrush, the Line6 DM-4, the Boss DD-20 and the
Boss RC-20.

I want to be able to play a guitar part, loop it, play another part over
that part and have the original loop fade out as I record another loop.
Essentially, record part A, loop it, play/record part B, loop it, play part
C and, after part A has faded out, loop part C. I do not care too much 
about
adding any crazy sounds effects to the mix, though a bit of delay would be
nice to be able to add into the mix.

Seems like the DD-20 is too limited as far as number of looping tracks as
well as how it saves the tracks. Seems like the RC-20 is too limited as far
as layering multiple tracks. The Line6 sounds poorly constructed and the
Headrush is something of an enigma.

Thoughts?

Thanks in advance for your time and assistance.

Carl McClatchey