i own one of each and i'm very glad i made the choice to buy both (although i didn't buy them at the same time. :lol: )
both are a joy to work on when piecing. in fact, i bought my 6600p because piecing on the 11000 was such bliss, i couldn't bear to set that task aside to use the 11000's embroidery feature.
i much prefer the 6600p for straight-line and soft curve quilting. i also like it better for those times i'm brave enough to practice my freemotion.
when i want to add fancy shmancy quilting motifs, the 11000 is my first choice. obviously, it's also the one i pull out when i want to do machine embroidery - including letters and words.
on days when i'm particularly ambitious i can have both out. the 11000 embroiders/quilts while i'm also using the 6600 for piecing/quilting.