I've only had one Ultra Lace piece. I am over a year with it. Not much difference from what I would get with the SFS.

Alicia, from Alicia Hair Systems in Anaheim was very impressed with the Ultra Lace. The base is really transparent. I've been impressed with it as well.
My wife says it's completely natural and no one would be able to tell anything.
There wasn't any worry with the SFS either but I do feel the Ultra Lace is a slight notch above.

With regards to density. You pretty much have to have a light density with the Ultra Lace. I think the hairline is a bit better.. maybe due to the light density combined with the lace being so extremely transparent.