I sent in my offer to purchase a Sweet Sixteen yesterday (will pick up Mollie on Friday).
The Sweet Sixteen and the Babylock Tiara are the same machine, one being branded for Babylock. Owners of both are on a very helpful Yahoo group which I've been lurking on.
I think the biggest difference between the Sweet Sixteen (HQSS) and the Tin Lizzie Sit Down (TLSD) is the way you orient yourself to the machine. The HQSS has you sitting facing the head of the machine and moving the fabric easily side to side. The pictures I've seen of the TLSD have you oriented similar to a domestic sewing machine so moving fabric to the right, you run up against the side of the machine, but have much more space that you did with a domestic due to the larger harp.
I've been pondering this decision for about 3 years now and am very excited for Friday afternoon.
Cheers, K